several embodiments of the present invention will be described with reference to the accompanying drawings . referring to fig1 - 5 , a motorcycle m has front and rear wheels wf , wr which are suspended from a frame f of the motorcycle m via front and rear pneumatic suspension units sf , sr , respectively . the front pneumatic suspension unit sf is provided with a first air pressure sensor s 1 capable of sensing the air pressure therein , and the rear pneumatic suspension unit sr is provided with a second air pressure sensor s 2 capable of sensing the air pressure therein . a speedometer ms of the motorcycle m is incorporated with a vehicle speed sensor ss ( see fig3 ). the rear pneumatic suspension unit sr comprises two buffer air cylinders sr 1 , sr 2 which are connected to the second air pressure sensor s 2 via communication conduits c 1 , c 2 , respectively , as shown in fig2 . the second air pressure sensor s 2 is connected via a line c 3 to a signal processor 13 which will be described later . each of the first and second air pressure sensors s 1 , s 2 comprises a pressure switch , and is adapted to generate an output signal of a high level when the air pressure sensed is higher than a predetermined level . the relation between output levels of these air pressure sensors s 1 , s 2 and variations in air pressures is shown in the following table . table 1______________________________________ output level of air pressureair pressure sensors______________________________________not less thanpredetermined level highlower thanpredetermined level low______________________________________ the vehicle speed sensor ss , which is shown in fig3 , includes a speedometer ms , an oscillatory plate 2 oscillated in accordance with the movement of a shaft 1 of a pointer of the speedometer ms , a photoelectric means 3 mounted close to a peripheral portion of the oscillatory plate 2 , and an inverter circuit 4 for inverting the output from the photoelectric means 3 . the oscillatory plate 2 has a recess 5 in a peripheral portion thereof which has an angular extent corresponding to a certain angle of oscillation in turn corresponding to a certain range of vehicle speeds less than a predetermined value ( for example , 16 km / h ). the photoelectric means 3 is provided with a light emitter 6 and a light receiver 7 . the photoelectric means 3 is adapted such that , when the vehicle speed is less than a predetermined level , the light from the light emitter 6 passes through the recess 5 to reach the light receiver 7 , and when the vehicle speed is greater than a predetermined level , the light from the emitter 6 is intercepted by a non - recessed section of a peripheral portion of the oscillatory plate 2 due to the oscillation thereof in accordance with an increase in the vehicle speed so that the light does not reach the light receiver 7 . when the light from the light emitter 6 does not reach the light receiver 7 , the output from the photoelectric means 3 is at a low level so that a signal of high level is produced at the output of inverter circuit 4 which indicates that the vehicle speed is not less than a predetermined level . the relation between the level of the output from the inverter circuit 4 , i . e . the level of the output from the vehicle speed sensor ss and variations in the vehicle speed is shown in the following table . table 2______________________________________ output level of vehiclevehicle speed speed sensor______________________________________lower thanpredetermined level lownot less thanpredetermined level high______________________________________ referring to fig5 output signals from the first and second pressure sensors s 1 , s 2 are fed to the inputs of a first and circuit 8 , and the output signal from the first and circuit 8 is inverted in an inverter 9 , and fed to a second and circuit 10 together with the output signal from the vehicle speed sensor ss . the output signal from the second and circuit 10 is fed to the input of a timer circuit 11 . an output signal produced by the timer circuit 11 causes an alarm or indicating device 12 to be actuated . the indicating device 12 comprises a lamp or a buzzer , which is mounted in the dashboard on the motorcycle m . the first and second and circuits 8 , 10 , inverter 9 , and timer circuit 11 constitute the signal processor 13 of the present invention . reference numeral 14 denotes a main switch , and 15 a battery to complete the circuit . when the vehicle speed is below a predetermined level , the output from the vehicle speed sensor ss is at a low level . accordingly , the output from the second and circuit 10 is at a low level so that the indicating device 12 is not actuated irrespective of the output levels of the first and second pressure sensors s 1 , s 2 . when both of the air pressures in the front and rear pneumatic suspension units sf , sr are not less than a predetermined level , both of the outputs from the first and second air pressure sensors s 1 , s 2 are at a high level . accordingly , the output from the first and circuit 8 , into which the outputs from the first and second air pressure sensors s 1 , s 2 are fed , is at a high level . since the output from the first and circuit 8 is inverted by the inverter 9 to a low level , the output from the second and circuit 10 is a low level . therefore , the indicating device 12 is not actuated irrespective of the output level of the vehicle speed sensor ss . when the vehicle speed exceeds a predetermined level and the air pressure in one or both of the front and rear pneumatic suspension units sf , sr is below a predetermined level , the output of the vehicle speed sensor ss is at a high level , and the output from one or both of the first and second air pressure sensors s 1 , s 2 is at a low level . therefore , an output signal of a low level is generated in the first and circuit 8 , and an output at a high level is produced by the inverter 9 . the output of high level from the inverter 9 and the output of high level from the vehicle speed sensor ss are simultaneously fed to the second and circuit 10 so that an output of high level is generated therein . the output of high level from the second and circuit 10 causes the timer circuit 11 to be energized . after a certain period of time ( for example , 6 seconds ) has elapsed , an output of high level generated in the timer circuit 11 will actuate the indicating device 12 . for example , when the motorcycle is running on a bumpy road , the air pressures in the first and second pneumatic suspension units sf , sr , which are generally kept not less than a predetermined level , are momentarily lowered below a predetermined level due to the buffer effect of these suspension units sf , sr thereby to cause the second and circuit 10 to generate an output of high level which is fed into the timer circuit 11 . however , this input time is too short to exceed the delayed time set by the timer circuit 11 so that the output from the timer circuit 11 remains at a low level . accordingly , the indicating device 12 is not actuated . fig6 shows another embodiment of the present invention , in which the same reference numerals and symbols are used for the parts corresponding to those of the first embodiment . when the main switch 14 is closed , the light emitted from the light emitter 6 reaches the light receiver 7 ( in the case where the vehicle speed is lower than a predetermined level ) so that transistors q 1 , q 2 are saturated . as a result , a transistor q 3 is cut off . an electric current flows to the base of a transistor q 4 through a resistor r 1 to saturate the transistor q 4 and allow the output of the vehicle speed sensor to be grounded . consequently , an output signal of low level is generated in the vehicle speed sensor ss . in the meantime , a capacitor c 1 in the signal processor 13 begins to be charged due to a resistor r 3 connected therein . however , the voltage of the capacitor c 1 does not reach a predetermined level until a predetermined period of time has elapsed , so that the base potential of a transistor q 5 is not sufficiently low . accordingly , transistors q 5 , q 6 are kept cut off , to allow a transistor q 7 to be saturated . thus , an electric current flows to the indicating device 12 . when a predetermined period of time has elapsed after the capacitor c 1 begins to be charged , the base potential of the transistor q 5 is suffieiently lowered so that the transistor q 5 is saturated . then , an electric current flows to a capacitor c 2 through a resistor r 8 , transistor q 5 and resistor r 4 to start charging the capacitor c 2 . the electric current further flows to the base of the transistor q 6 through the resistor r 5 so that the base potential thereof is increased to saturate the transistor q 6 . as a result , the base potential of the transistor q 7 is lowered to cut off the transistor q 7 , and an electric current does not flow to the indicating device 12 . thus , the indicating device stops giving an indication . by virtue of the above - described structure , an electric current is automatically supplied to the indicating device 12 for only a predetermined period of time after the main switch 14 has been closed . therefore , faults in the indicating device 12 and the circuit for actuating the indicating device 12 can be detected each time the main switch 14 is operated . when the vehicle speed exceeds a predetermined level , the light from the light emitter 6 is intercepted or does not reach the light receiver 7 so that the transistors q 1 , q 2 are cut off . as a result , an electric current flows to the base of the transistor q 3 through a resistor r 0 to increase the base potential of the transistor q 3 . thus , the transistor q 3 is saturated . consequently , the base potential of the transistor q 4 is lowered so that the transistor q 4 is cut off to allow an output of high level to be generated in the vehicle speed sensor ss . when the air pressures in the front and rear pneumatic suspension units sf , sr are above a predetermined level and the switches for the first and second air pressure sensors s 1 , s 2 are closed , the base potential of the transistor q 5 is kept at a low level so that the transistor q 7 is cut off . consequently , an electric current does not flow to the indicating device 12 . when the air pressure in one or both of the front and rear pneumatic suspension units sf , sr is less than a predetermined level with the switch for one or both of the first and second air pressure sensors s 1 , s 2 opened , in the circumstance that the vehicle speed exceeds a predetermined level with the output signal from the vehicle speed sensor ss at a high level , the capacitor c 1 starts discharging through the resistors r 2 , r 3 . after a predetermined period of time has elapsed , the base potential of the transistor q 5 is increased so that the transistor q 5 is cut off . as a result , the capacitor c 2 starts discharging through the resistors r 5 , r 6 . when the base potential of the transistor q 6 is lowered , the transistor q 6 is cut off so that an electric current flows to the base of the transistor q 7 through a resistor r 7 . when the base potential of the transistor q 7 is thus increased , the transistor q 7 is saturated . as a consequence , an electric current flows to the indicating device 12 to energize the same to provide an indication . in the above embodiments , the outputs from the first and second air pressure sensors s 1 , s 2 are fed to the signal processor 13 . however , one of the air pressure sensors s 1 , s 2 may be omitted . namely , the output from only one of the air pressure sensors s 1 , s 2 may be fed to the signal processor 13 . as has been described in detail above , the present invention permits actuating the indicating device to give an indication when the air pressures in the pneumatic suspension units are lower than a predetermined level with the vehicle speed continuing to exceed a predetermined level for more than a predetermined period of time . namely , the present invention contemplates sensing the air pressures in the pneumatic suspension units and if they continue to be below a level in a predetermined range while the vehicle is running at a speed in excess of a predetermined level , this condition is conveyed to the rider or driver . therefore , the rider or driver is immediately informed of the necessity of repairing the pneumatic suspension units . in fact , the device according to the present invention is very useful to maintain the shock absorbing functions of the pneumatic suspension units in a normal condition . moreover , the indicating device is not actuated in response to a momentary decrease in air pressure in the pneumatic suspension units which occurs while the vehicle is traveling , for example , on a bumpy road . in other words , the indicating device can be prevented from being erroneously actuated . since the signal processor provided in the device according to the present invention has a fault - checking circuit which permits automatically supplying electric current to the indicating device for a predetermined period of time when starting the signal processor , faults in the indicating device and the circuit for actuating the indicating device can be checked each time the main switch is operated . since the indicating device is adapted to be actuated when the vehicle speed is in excess of a predetermined value with the air pressure in one or both of the front and rear pneumatic suspension units continuing to be below a predetermined level for more than a predetermined period of time , suitable air pressures in the front and rear pneumatic suspension units can be accurately checked while the vehicle is running on a road with the total weight thereon acting on the vehicle . moreover , the device according to the present invention prevents the indicating device from being erroneously actuated even when variations in air pressures in the pneumatic suspension units occur due to shock absorbing actions thereof while the vehicle is running on the road . the present invention is not , of course , limited to the above - described embodiments and it may be modified in various ways withing the scope of the appended claims .