hereafter , the invention is described by way of preferred embodiments shown in the drawings , in which the component parts having correspondences in the basic configuration of fig1 are designated by the same reference numerals . referring to fig3 there is illustrated an example of a truck incorporating the fuel supply for gaseous fuel operated vehicle according to the invention . as shown in this figure , the truck 20 can be divided into an engine room section 6 for accommodating an engine and associated parts , a driver &# 39 ; s room section 7 with a seat , and a rear freight room section 3 with a loading platform . the fuel gas bomb 2 is formed , for example , from a metal and fixed in the freight room section 3 by the use of a belt and bolts . the bomb 2 is charged with a compressed fuel gas , for example , under pressure 150 - 200 kg / cm 2 in freshly charged state . this fuel gas is led out through a bomb valve 11 which is fixedly clamped to the bomb 2 . indicated at 12 is a high pressure conduit which connects the bomb valve 11 with a primary ( upstream ) regulator 1 which is fixed in the freight room section 3 in a manner similar to the bomb 2 , and which pays sufficient considerations to safety since it is subjected to substantially the same pressure as the bomb 2 . the primary regulator 1 is a decompressing valve employing a diaphragm as will be described hereinlater , regulating the fuel gas substantially to a constant pressure level , for example , to a level of 5 to 6 kg / cm 2 when the bomb 2 is filled with more than a predetermined amount of fuel gas . denoted at 9 is a fuel cutoff valve which is formed integrally with the primary regulator 1 , and which is constituted , for example , by a high pressure electromagnetic valve and associated with an ignition switch or the like to cut off the supply of fuel gas when the vehicle is at rest or at the time of emergency . on the other hand , accommodated in the engine room 6 is a secondary ( downstream ) regulator 4 , an engine 10 and a mixer 5 which is attached to the engine . the secondary regulator 4 and mixer 5 are interconnected by a lo pressure hose 13 of rubber or similar material . similarly to the primary regulator 1 , the secondary regulator 4 consists of a decompressing valve with a diaphragm , for reducing the atmospheric pressure the fuel gas which is fed from the primary regulator 1 through a low pressure conduit 8 extended across the driver &# 39 ; s cabin section 7 , for example , in a lower portion of the vehicle body in that section . after decompression by the secondary regulator 4 , the fuel gas is led into the aforementioned low pressure hose 13 . when the vehicle operates exclusively on cng , the mixer 5 may be incorporated into the engine 10 in a manner similar to a carbureter of the gasoline engine . however , in a case where the vehicle uses gasoline in addition to cng , it is desirable to mount the mixer 5 in the vicinity of a carbureter , not shown , or to form the mixer 5 integrally with a cabureter , sucking out the fuel gas from the downstream regulator 4 through the low pressure hose 13 in an amount corresponding to the amount of induction air by the vacuum which is generated by a component like a venturi nozzle . the low pressure hose 13 is desired to be as short as possible to cope with abrupt changes in the required amount of fuel . in a case where the vehicle also operates on gasoline , the operation of the fuel cutoff valve , changeover of the fuel system , fuel supplying operation and resetting of the spark advance characteristics due to the change of fuel may be controlled by an electronic circuit such as a micro - processor or the like . referring to fig4 there is illustrated an example of a passenger car incorporating the present invention , in which those parts common to the basic arrangement of fig1 and the truck of fig3 are designated by the same reference numerals . in this case , the passenger car 20 &# 39 ; can be divided into an engine room section 6 &# 39 ; accommodating an engine and associated parts , a passenger &# 39 ; s room section 7 &# 39 ; with seats , and a luggage room section 3 &# 39 ; including a trunk room or the like . the arrangement , construction and operation of the fuel supply system of the invention are same as in the case of the truck 20 of fig3 and therefore the description in these aspects is omitted here to avoid repetitions . referring to fig5 there is shown in section an embodiment of the regulator which is suitable for use in the fuel supply system of the invention , particularly for use as the primary regulator of the system . the primary regulator 1 includes a high pressure casing or housing 14 which is attached with a fuel cutoff valve 9 , a low pressure casing or housing 16 which is assembled with decompressing parts , and a ptc heater 15 interposed between the two casings 14 and 16 . the high pressure casing 14 interiorly defines a fuel cutoff chamber 17 which is in communication with a first high pressure gas passage 19 connected to a high pressure conduit from a bomb , not shown , and with a second high pressure gas passage 23 leading to a decompressing chamber 21 formed in the low pressure casing 16 . the fuel cutoff valve 9 is provided with a fuel cutoff needle valve 25 which is movable up and down in the fuel cutoff chamber 17 , and an electromagnetic coil , not shown , for driving the needle valve 25 , and fixed to the high pressure casing 14 by screws 27 . the electromagnetic coil is connected to a battery 31 through a switch 29 which is operationally linked to an ignition switch , not shown , driving the needle valve 25 downward into abutting engagement with a conical wall 33 in a lower portion of the fuel cutoff chamber 17 to block the downward flow of the fuel , for example , when the engine is at rest . the switch 29 is not necessarily required to be a mechanical switch , and may be constituted by an electronic circuit with a switching function , for example , a function of turning on an off in response to a signal of an emergency sensor or the like . formed in a lower portion of the high pressure casing 14 is a valve chamber 35 in which a valve 37 is loosely fitted . the valve 37 has a non - circular shape in section with protuberances or recesses on its circumference as shown in fig6 and operates to block the downward flow of the fuel when it is in intimate contact with a terminal open end 38 of the second high pressure passage 23 , and to spout the fuel gas into the decompressing chamber 21 through the gap spaces formed between the protuberances or recesses of the valve 37 and the inner periphery of the valve chamber 35 when it is displaced downwardly away from the terminal open end 38 of the second high pressure passage 23 . the lower end of the valve 37 is abutted against an l - shaped lever 41 which is pivotally supported on a pin 39 fixed in the low pressure casing 16 . this l - shaped lever 41 is connected to a shaft 45 which transmits the displacement of a diaphragm 43 which defines the decompressing chamber 21 together with the low pressure casing 16 . the shaft 45 is fixed by a nut 49 to shells 47 a and 47b which grip the diaphragm 43 . designated at 51 is a spring member which is interposed between the outer shell 47a and a cover 53 which is fixed to the low pressure casing 16 through the diaphragm 43 and provided with a hole opening to the atmosphere . the spring constant of this spring member is determined depending upon the desired fuel gas pressure after the pressure reduction . when the pressure in the reducing chamber 21 is low and the diaphragm 43 is displaced rightward in the drawing , the valve 37 is held in the lower position , permitting the fuel gas to flow into the decompressing chamber 21 through the second high pressure gas passage 23 . as the air pressure in the decompressing chamber 21 gradually increases , the diaphragm 43 is displaced leftward , and the valve 37 closes the terminal open end 38 of the second high pressure passage 23 to block the flow of fuel gas . in this manner , the air pressure in the decompressing chamber 21 is constantly maintained at a level corresponding to the spring force of the spring member 51 at a stationary position of the diaphragm 43 , discharging the fuel gas into the low pressure gas passage 55 under that pressure . on the other hand , the fuel gas undergoes adiabatic expansion as it is released into the decompressing chamber 21 , so that the parts neighboring the terminal open end 38 of the second high pressure passage 23 and the valve chamber 35 are cooled to a considerable degree . therefore , in present embodiment , a ptc heater 15 is provided around the circumference of the second high pressure passage 23 and the valve chamber 35 . the ptc heater 15 which is fixedly mounted on the low pressure casing 16 by bolts 63 and 65 includes a heating element 57 consisting of a ptc thermistor of batio 3 type , wires 59 connected to the opposite ends of the heating element 57 to apply a predetermined voltage thereto , and a waterproof cover 61 . the voltage application to the heating element 57 may be effected without a special control , for example , linked with the ignition to effect constantly when the engine is in operation . although the temperature in the neighborhood of the terminal open end 38 drops immediately after starting the engine , the neighboring parts are heated up almost instantly by the large current flowing through the heating element 57 which initially has a small resistance . once heated up , the resistance of the heating element 57 increases extremely , maintaining the neiborhood of the second high pressure passage 23 and valve chamber 35 at a predetermined normal temperature without a large electric power consumption . though the ptc heater 15 is located only around the second high pressure gas passage 23 and valve chamber 35 in this particular embodiment , it should be understood that the present invention covers various modifications or alterations which can be made in this regard , for example , location of the heater in other positions or extension of the heater onto the circumference of the decompressing chamber 21 . referring now to fig7 there is schematically shown another embodiment of the fuel supply system for gaseous fuel operated vehicle , employing a regulator with a heat pipe . in this case , the regulator is divided into two or more units to shorten the length of the high pressure gas conduit . if it is divided into two units , for example , a primary regulator 1 which is formed integrally with a fuel cutoff valve 9 and a cng charged bomb 2 are located in a luggage room section in the rear portion of a vehicle ( a trunk room of a passenger car or a loading platform of a truck ), locating a secondary regulator 4 in an engine room section 6 in a front portion of the vehicle along with a mixer 5 and an engine 10 . the primary and secondary regulators 1 and 4 are interconnected by a conduit 8 which is laid across a passenger &# 39 ; s room section 7 of the vehicle . the fuel gas such as cng which is charged in the fuel bomb 2 under pressure of 150 - 200 kg / cm 2 in a fresh state is fed to the primary regulator 1 through the fuel cutoff valve 9 to reduce the gas pressure , for example , to 3 - 5 kg / cm 2 , and then fed to the secondary regulator 4 through the conduit 8 . the fuel gas which has undergone the primary pressure reduction is further decompressed by the secondary regulator 4 substantially to the level of the atmospheric pressure , for example , to a level of 0 to 200 mmaq , and supplied to the engine 10 after mixing same with air in a predetermined ratio by the mixer 5 which is formed integrally with or separately from the engine 10 . the exhaust gas resulting from combustion of the air - fuel mixture in the engine 10 is cleaned by a catalytic converter 66 before releasing into the atmosphere through an exhaust pipe 67 . illustrated in fig8 is the positional relationship of the various component parts of a vehicle as shown in fig7 a truck in this case . like component parts are designated by like reference numerals , and their description is omitted here to aboid repetitions . in this embodiment , the primary regulator 1 is constituted , as shown particularly in fig9 by a high pressure casing 68 which is attached with the fuel cutoff valve 9 , a low pressure casing 69 which is provided with various parts for pressure reduction , and a heat pipe 70 which is interposed between the two casings and has its one end extended into the exhaust pipe 67 . the high pressure casing 68 interiorly defines a fuel cutoff chamber 71 which is in communication with a first high pressure gas passage 72 connected to a high pressure conduit from the fuel bomb , not shown , and with a second high pressure gas passage 74 communicating through the valve chamber 73 with a pressure reducing chamber ( not shown ) which is formed in the low pressure casing 69 . the fuel cutoff valve 9 includes a needle valve 74 &# 39 ; which is movable up and down in the fuel cutoff chamber 71 , and an electromagnetic coil , not shown , for driving the needle valve 74 &# 39 ;, the cutoff valve 9 being integrally attached to the high pressure casing 68 by fastening means or the like . by making or braking the connection of the electromagnetic coil with a battery , not shown , the needle valve 74 &# 39 ; is driven downward in the drawing to abut against a conical wall 75 in a lower portion of the fuel cutoff chamber 71 to block the downward flow of the fuel when the engine is at rest or at the time of emergency . the low pressure casing 69 is of a known construction using a diaphragm , not shown , and arranged to move up and down a valve 76 , which is loosely fitted in the valve chamber 73 , according to displacement of the diaphragm thereby opening or closing a clearance between the valve 76 and an annular projection 77 ( corresponding to the terminal open end ) which is formed on the high pressure casing 68 around the circumference of the opening of the second high pressure gas passage 74 to control the fuel gas flow from the second high pressure gas passage 74 to a decompressing chamber which is formed in the low pressure casing 69 though not shown , while reducing the fuel gas pressure to a predetermined level . on the other hand , the exhaust manifold 78 of the engine 10 is connected to a catalytic converter 66 through an upstream exhaust pipe 79 , and an exhaust pip 67 is connected to the catalytic converter 66 . the exhaust gas which is discharged from the respective cylinders of the engine 10 and led to the upstream exhaust pipe 79 through the exhaust manifold 78 is cleaned through the catalytic converter 66 and passed in hot state through the exhaust pipe 67 before release into the atmosphere . the heat pipe 70 has its one end so arranged as to wrappingly contact the high pressure casing 68 around the circumference of the second high pressure gas passage 74 , forming an annular cavity 80 in concentric relation with the second high pressure gas gassage 74 . the other end 81 of the heat pipe 70 is protruded into the exhaust pipe 67 through a flange 82 which is formed on the exhaust pip 67 . the heat pipe 70 is fixed to the flange 82 by way of a disk - like projection 83 which is formed around the heat pipe 70 and fastened to the flange 82 by a plural number of bolts and nuts 85 through a heat resistant shock absorbing material 84 like rubber of the like . as seen in fig1 which shows the basic construction , the heat pipe 70 is constituted by a pipe 87 having a wick structure with high capillarity adhered on the inner periphery thereof , and an operating liquid which is sealed in the pipe after evacuation such that the wick is filled with the liquid to a sufficient degree . accordingly , when one end of the heat pipe is heated while cooling the other end , the operating liquid in a heated zone is evaporated by depriving the heat of vaporization , and the operating liquid in a cold zone condenses by giving up the heat of condensation . as a result , there occurs a pressure difference between the vapor phases in the heated and cooled zones , causing the vapors to flow from the heated zone to the cooled zone through the passage 88 in the pipe 87 . on the other hand , the liquid surface in the heated zone sinks into the wick 86 by evaporation of the operating liquid , increasing the capillary pressure , while the capillary pressure in the cooled zone drops due to condensation of the vapors . consequently , by the difference in capillary pressure between the heated and cooled zones , the operating liquid in the cooled zone is caused to flow toward the heated zone through the wick . in this manner , the vapors resulting from evaporation in the heated zone are condensed in the cooled zone and returned to the heated zone , and the same cycle is repeated . thus , heat is transferred to the cooled zone by vapors , mainly in the form of latent heat resulting from phase changes by evaporation and condensation , dissipating the heat of condensation to the outside from the cooled zone . normally , the amount of heat transfer by a heat pipe is about several hundred times that of copper . in this embodiment , the aforementioned heated zone corresponds to the heat pipe portion which is disposed in the exhaust pipe 67 , and the cooled zone corresponds to the circumference of the second high pressure gas passage 74 of the primary regulator 1 and the valve chamber 73 . accordingly , the heat of the exhaust gas which passes through the exhaust pipe 67 is tranferred to the regulator 1 adequately through the heat pipe 70 , suppressing the cooling efect of the adiabatic expansion of the fuel gas from the second high pressure gas passage 74 to prevent freezing of fuel gas components . although the heat pipe 70 is located only around the circumferences of the second high pressure gas passage 74 and valve chamber 73 in this particular embodiment , it is possible to locate the heat pipe in other positions without departing from the technical sphere of the invention , for example , to extend the heat pipe as far as the circumferences of the fuel cutoff chamber 71 and the decompression chamber in the low pressure casing 69 . the other end of the heat pipe 70 which is protruded into the exhaust pipe 67 downstream of the catalytic converter 66 may be instead protruded into the upstream exhaust pipe 79 or the catalytic converter 66 if desired . it will be appreciated from the foregoing description that , according to the present invention , the fuel gas which flows through the conduit laid in the passenger &# 39 ; s room section has already been decompressed to a substantial degree by the upstream regulator , almost completely precluding the possibility of leakage of fuel gas from the conduit . in addition , since the upstream regulator is located in the vicinity of the fuel gas bomb in the luggage room section , the length of the high pressure gas conduit can be shortened to an extent which is desirable for the sake of safety . namely , should a leak of the high pressure fuel occur , it takes place in the luggage room section of the vehicle and the leaked gas is released into the atmosphere safely without flowing into the passenger &# 39 ; s room section or the engine room section from which an inflammable gas should be kept out . in a case where the fuel cutoff valve is provided between the fuel gas bomb and the upstream regulator , the supply of fuel gas is cut off at a position close to the bomb when the engine is stopped . this minimizes the conduit portion which is constantly subjected to a high pressure , enhancing the reliability and safety further more . since the cooling effect of the adiabatic expansion of the decompressed fuel gas on the circumventive parts is precluded by a ptc heater in an embodiment of the regulator according to the invention , it is possible to heat up these parts more promptly immediately after starting an engine and to maintain them at a desired temperature more stably thereafter as compared with the technology of heating the regulator by the use of cooling water heated by the engine . this heat is also transferred to the fuel cutoff valve to prevent freezing thereof . in case of the regulator employing a heat pipe , an arbitrary portion of the exhaust system can be used as a heat source , so that the temperature of the heated zone can be set at a desired level without providing a special energy source for heating . the intermediate portion between the opposite ends of the heat pipe normally has a heat insulating structure , which has no heating effect on adjacent parts even if located along a steel sheet of the vehicle body or along the fuel gas bomb , and therefore imposes no thermal affect on the gas fuel supply system .