PURPOSE: To prevent malfunctioning of an air bag even if impact is applied by mistake to the bag during inspection or repair by providing a manually stopping means for manually stopping a drive means for an air bag actuation means.

CONSTITUTION: When a switch 14 is on, the outputs of a logic circuit 15 are always at high level and a faile-safe relay 10 is turned off, so that even if a mechanical acceleration sensor 9 and a piezoelectric element 1 are ready to ignite a detonator 7, the detonator 7 cannot be ignited. Therefore, even if impact is applied by mistake to the piezoelectric element 1 or the mechanical acceleration sensor 9 during repair or inspection of a collision detection device of a vehicle, a current flows through the detonator 7 with the switch 14 being turned on and an air bag is prevented from being unfolded. When the switch 14 is turned on, a lamp driving circuit 17 is actuated to turn on a lamp 18, thus informing a driver that the air bag will not unfold. In a normal running state the driver turns off the switch 14 so that in the case of a collision the air bag can be opened.