an embodiment of the invention will be explained below with reference to the attached drawings . the invention , however , is not limited to this embodiment . as shown in the figure , the module 1 includes a ceramic wiring board 2 forming a base portion of the module 1 , a frame 3 soldered to the ceramic wiring board 2 in such a way as to surround lsis ( electronic circuit parts ) 20 , which are mounted on this ceramic wiring board 2 , and a cover 4 that covers an opening portion 30 of this frame 3 . the lsi 20 is fixed to the ceramic wiring board 2 via a plurality of solder balls 21 having a diameter on the order of tens of microns . moreover , a cooling fin 22 is arranged on the upper surface of the lsi 20 . springs ( not shown ) are provided between the cooling fin 22 and a cooling material 49 , and the cooling fin 22 is kept in close contact with the lsi 20 by the forces of the springs . at the top end portion of the frame 3 , a flange section 31 extending horizontally is provided . this flange section 31 is formed with a groove 33 , in which a heat - resistant sealing material 32 is disposed to assure airtightness with the cover 4 , and screw holes 34 for fastening screws 40 to secure the cover 4 to the frame . a groove 43 is formed in the periphery of the cover 4 , which corresponds to the groove 33 in the flange section 31 of the frame 3 . screw holes 44 are also provided in the periphery of the cover 4 , which correspond to the screw holes 34 . furthermore , two sealing holes 45 are formed in the periphery further inside than the groove 43 , which serve as inlets to charge a liquid coolant . these sealing holes 45 are 10 each provided with screw grooves and flanges . after the module is filled with the liquid coolant , the module 1 can be tightly sealed by pressing o - rings 47 attached to the flange with the sealing screws 46 . furthermore , an air - cooling heat sink 48 is bonded to the top surface of the cover 4 . a cooling material 49 having fins complementary to the shape of the cooling fin 22 is bonded to the lower surface of the cover 4 . a chamber used to fill the module 1 with the liquid coolant will be explained below . fig2 illustrates a state of the module 1 placed in the chamber 5 that can be hermetically sealed . as shown in the figure , the chamber 5 includes an xy table 6 that positions and fixes the module 1 , a fastening unit 7 that closes the sealing holes 45 of the cover 4 of the module 1 by means of the sealing screws 46 , a filling unit 8 that fills the module 1 with the liquid coolant through the sealing holes 45 of the cover 4 , a gas supply unit 9 that supplies an inert gas and an oil - sealed rotary vacuum pump 10 that deaerates the inside of the chamber 5 . the xy table 6 is a well - known movable table provided movably in the two orthogonal directions on a horizontal plane ( x , y directions ). the xy table 6 has feed screws provided in the x , y directions , a motor that rotates these feed screws , an encoder and a table that moves according to these feed screws and guides ( both not shown ). a set jig 60 is provided on the xy table 6 to position and fix the module 1 . a chuck 61 is provided in the chamber 5 to grip the sealing screw 46 described above . the fastening unit 7 includes a motor - driven driver 72 that has a driver bit 71 corresponding to the sealing screw 46 , and a lifting device 74 that moves this driver 72 up and down along a guide 73 . the driver 72 is provided with a torque controller 75 that adjusts a torque for fastening . the coolant filling unit 8 includes a syringe 82 having a supply nozzle 81 , which can be inserted into the sealing hole 45 , and a lifting device 84 that moves this syringe 82 up and down along a guide 83 . the syringe 82 is provided with a piston 85 and a valve 86 so as to adjust the volume of the liquid coolant supplied from the supply nozzle 81 . opening this valve 86 allows the liquid coolant to be supplied from the supply nozzle 81 by the self weight of the piston 85 and a pressure difference between atmospheric pressure and the inner pressure of the decompressed chamber 5 , as will be described later . the gas supply unit 9 includes a gas bomb 91 filled with compressed nitrogen gas for pressure adjustment and a pipe 92 that leads the nitrogen gas in this gas bomb 91 into the chamber 5 . the pipe 92 is provided with a flow rate control valve 93 that controls the flow rate of the nitrogen gas supplied . next , the procedure for sealing the liquid coolant into the above described module 1 will be explained with reference to the flow chart in fig3 and fig2 and 4 . sealing the liquid coolant into the module 1 is carried out at ordinary temperatures . in step 1 , as shown in fig2 the module 1 before sealing ( before filling it with the liquid coolant ) is positioned and fixed by the set jig 60 on the xy table 6 disposed in the chamber 5 . at this time , the frame 3 and cover 5 are fastened together with the fastening screws 40 . as shown in fig4 the pressure inside the chamber 5 and the module 1 at this time is atmospheric pressure p 0 . moreover , the syringe 82 of the coolant filling unit 8 has beforehand been filled with the liquid coolant c . as the liquid coolant c , it is preferable to use thermal conductive , insulating and heat resistant oil . in this embodiment , poly α olefin is used . it is also preferable to use the liquid coolant c well deaerated using a deaerator ( not shown ) such as a deaerating chamber and stirrer beforehand . the amount of the liquid coolant c to be filled , since the module 1 is normally used in an upright position , is set to take a level at which a lsi at the top of this module in the upright position and the cooling fin attached thereto are almost immersed in the liquid coolant . accordingly , the nitrogen gas is also contained in the sealed module 1 . this nitrogen gas serves to prevent the oxidation of the liquid coolant c , absorb the volume expansion of the liquid coolant c when the volume of the liquid coolant c expands and prevent the deformation of the frame 3 or defective coupling between the frame 3 and the ceramic wiring board 2 . the volume ratio of the liquid coolant c to the nitrogen gas after filling and sealing of the liquid coolant c in the inner space of the module 1 is set , for example , to 8 : 2 . then , in step 2 , the vacuum pump 10 of the chamber 5 is operated and the pressure p 1 inside the chamber 5 is reduced from atmospheric pressure p 0 to an approximate vacuum pressure p 1 ( see fig4 ). the pressure p 1 inside the chamber 5 , though it depends on the capacity of the vacuum pump , is from 0 . 1 to 0 . 2 torr in this embodiment , and the chamber 5 and module 1 are deaerated to an approximate vacuum condition . then , in step 3 , the vacuum pump 10 is stopped , the flow rate control valve 93 is opened and the nitrogen gas is introduced from the gas bomb 91 into the chamber 5 so that the pressure inside the chamber 5 becomes a predetermined coolant filling pressure p 2 . due to a pressure increase accompanying the introduction of the nitrogen gas , the pressure inside the chamber 5 and module 1 is increased to the coolant filling pressure p 2 . the coolant filling pressure p 2 is from 50 to 150 torr in this embodiment . in step 4 , under this coolant filling pressure p 2 , the supply nozzle 81 is lowered using the lifting device 84 and the end of the supply nozzle 81 is inserted into the sealing hole 45 of the module 1 . then , the valve 86 is opened and the liquid coolant c stored in the syringe 82 is filled into the module 1 through the sealing hole 45 . during this filling , since the pressure inside the module is lower than atmospheric pressure , the liquid coolant c is filled into the module by this pressure difference and the self weight of the piston . moreover , since the pressure inside the module 1 is higher than the vacuum pressure , this prevents the entry of air bubbles from the joints of the syringe 82 and nozzle 81 or the reverse spouting of the liquid coolant through the sealing hole 45 of the module 1 , enabling smooth filling while suppressing the mixture of air bubbles during filling to a minimum . then , in step 5 , after the filling of the liquid coolant c is completed , the flow rate control valve 93 is opened again and the nitrogen gas is introduced from the gas bomb 91 into the chamber 5 again so that the pressure inside the chamber 5 becomes a sealing pressure p 3 between the liquid coolant filling pressure p 2 and atmospheric pressure p 0 . due to a pressure increase accompanying the introduction of the nitrogen gas , the pressure inside the chamber 5 and module 1 is increased to the sealing pressure p 3 . this sealing pressure p 3 is set to such a value that the pressure inside the module 1 is lower than the atmospheric pressure p 0 even when the lsi 20 operates and its temperature increases to the allowable upper limit temperature , for example , 70 ° c . and the temperature of the liquid coolant c increases . in this embodiment , it is set from 400 to 500 torr . then , in step 6 , the module 1 is moved using the xy table 6 and positioned so that the sealing hole 45 comes right under the fastening unit 7 . subsequently , the driver bit 71 is lowered using the lifting device 74 . by running the motor 72 , the sealing screw 46 held by the chuck 61 is fastened into the sealing hole 45 using the driver bit 71 . at this time , the torque is adjusted by the torque controller 75 so that the sealing screw is fastened with a torque enough to deform the o - ring 47 . this fastening operation is repeated the number of times corresponding in number with the sealing holes 45 provided ( two in this embodiment ) by moving the module 1 using the xy table 6 . in this way , the module 1 is sealed hermetically and the sealing of the liquid coolant c into the module 1 is completed . then , in step 7 , the flow rate control valve 93 is closed to stop the supply of the nitrogen gas from the gas bomb 91 . a leak valve ( not shown ) of the vacuum pump 10 is opened to introduce external air into the chamber 5 . after ensuring that the pressure inside the chamber 5 has returned to the atmospheric pressure p 0 , the module 1 is taken out of the chamber 5 , and this completes the procedure . according to this embodiment , when filling the liquid coolant c into the module 1 , the pressure inside the chamber 5 is set to the filling pressure p 2 , which is a negative pressure , and therefore it is possible to minimize the generation of air bubbles during filling as compared with the conventional way of filling the liquid coolant c under atmospheric pressure . thus , this embodiment can improve the cooling performance of the module 1 and thereby the throughput of a computer incorporating this module , as comparison with the conventional way . moreover , the pressure inside the module 1 is set to be lower than the atmospheric pressure even when the lsi 20 operates and the temperature of the liquid coolant c sealed in the module 1 increases . accordingly , even if the volume of the liquid coolant c increases as the temperature increases , this will not cause the frame 3 to be deformed or result in defective coupling between the ceramic wiring board 2 and the frame 3 , making it possible to suppress the leakage of the liquid coolant c while the module 1 is operating . in the above embodiment , explained as a module in which a liquid coolant is to be sealed is the air - cooled module having the air - cooling heat sink 48 on the cover 4 . however , the method of sealing a liquid coolant into a module according to the invention is also applicable to a water - cooled module having a cooling water path provided in a module cover if it has a sealing hole that serves as the inlet . as the pressure adjustment gas , it is preferable to use nitrogen gas , which is an inert gas , as is in the embodiment . however , other inert gases such as helium , argon , neon or other gases , if they at least do not deteriorate the effects of the invention , may also be used . in the above described embodiment , two sealing holes are formed as the inlets , but the number of sealing holes may also be one or two or more . furthermore , the above described embodiment describes the case where sealing operation is carried out at ordinary temperatures , but it is possible to change the temperature for operation as far as it does not deteriorate the effects of the invention .