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.