when the ignition switch of the vehicle is turned on self diagnosis of the air bag is performed for a predetermined length of time there are various kinds of diagnoses first as a subroutine for diagnosing the non activation state of the safing g sensor 103 is started step s 1 the diagnostic control and processing section 33 in the microcomputer 3 outputs a pseudo crash signal g to the safing g sensor 103 step s 2 next the diagnostic control and processing section 33 acquires the output of the a d converter 21 from the asic 2 step s 3 and detects whether the acquired value is low or not step s 4 if the acquired value is low even when the pseudo crash signal g is output yes in step s 4 it is determined that the internal circuit of the sating g sensor 103 is faulty thus determining the fault state step s 5 on the other hand if the acquired value is high no in step s 4 this means that the circuit is functioning correctly and the situation is thus determined to be normal step s 6 after which the process is terminated step s 7 fig5 is a flowchart showing a procedure for operating the microcomputer 3 to check the operation of the a d converter 21 in the asic 2 when a subroutine for checking the operation is started step s 11 the diagnostic control and processing section 33 in the microcomputer 3 supplies a pseudo.