Acceleration data on acceleration that a vehicle undergoes are written
in a readable memory, the acceleration being detected by an acceleration sensor.
When an airbag is inflated, the time of the inflation is determined. Either acceleration
data on acceleration that the vehicle undergoes during a predetermined time period
before or after the determined time or acceleration data on acceleration that the
vehicle undergoes during two predetermined time periods before and after the determined
time are read from the readable memory, and the acceleration data read from the
memory are stored in a nonvolatile memory as a collision history.
