PURPOSE: To improve the reliability of collision detection of a vehicle by a simple circuit element by providing two circuit sides interconnected with three diode bridges and providing three collision sensors in a firing circuit for driving a passenger restraint device such as an air bag.

CONSTITUTION: A firing circuit 10 for developing a pair of air bags has two circuit sides 12, 14 connected with a direct current power supply 16 in parallel. Each circuit side 12, 14 has a normally open safety sensors 30, 38, explosive fuses 32, 40 which become a trigger for developing a first air bag when a current value thereof exceeds a threshold value, and normally open collision sensors 34, 42. A section between both circuit sides 12 and 14 is connected by a first to a third diode bridges 46, 54, 62 consisting of diodes 48, 56, 68 connected in reverse parallel. A third collision sensor 70 is connected between a branching point 72 between a third bridge 62 and the diode 68 and a branching point 74 between both circuit sides 12 and 14.