To achieve an air bag system in which a safing determination processing circuit can be implemented on one integrated circuit, the air bag system, which is equipped with first and second sensors (101, 103) for detecting a vehicle crash, and which outputs a signal for expanding an air bag when it is determined that a vehicle crash has occurred based on the outputs of the first and second sensors, comprises: a processing unit (3) for processing an output signal of the first sensor (101) by software; and a processing circuit (2) for processing an output signal of the second sensor (103) by hardware, wherein the processing unit (3) includes a non-activation fault diagnosis section (33) which, based on the output signal of the second sensor (103), detects a failure that can lead to non-activation of the air bag, and the processing circuit (2) includes an erroneous-activation fault diagnosis section (240) which, based on the output signal of the second sensor (103), detects a failure that can lead to erroneous activation of the air bag.