PURPOSE: To effectively cool the interior of an explosion-proof case by introducing the open air into the case.

CONSTITUTION: A suction/discharge pump unit 14, a gas sensor 21 and a solenoid changeover valve 22 are housed in an expolosion-proof case 12 and an open air introducing hole 23 is provided to the explosion-proof case 12 and the suction joint 16B of a discharge pump 16 is opened to the interior of the explosion-proof case 12. Therefore, when purge air is supplied from the discharge pump 16 in order to discharge vapor in a suction pipeline 13, a sensor case 20 and vapor piping 9, the air in the explosion-proof case 12 is sucked from the suction joint 16B to make the pressure in the explosion-proof case 12 negative and the open air is introduced into the explosion-proof case 12 through open air introducing piping 24 and an open air introducing hole 23 to cool the suction/discharge pump unit 14, gas sensor 21 and solenoid changeover valve 22 in the explosion- proof case 12.