An acceleration sensor detects an acceleration of a vehicle which is caused by a collision of the vehicle. A safety device protects an occupant in the vehicle from the collision of the vehicle. A trigger device triggers the safety device. The trigger device is activated in response to an output signal from the acceleration sensor. An operation of the trigger device is detected and a detection signal representative thereof is generated. A first judgment device judges whether or not the trigger device is activated on the basis of the detection signal. When the trigger device is judged to be activated, a second judgment device judges whether the activation of the trigger device is caused by a correct cause or a false cause on the basis of the output signal from the acceleration sensor. The judgment by the second judgment device is allowed only once and subsequent judgments by the second judgment device are inhibited. A result of the judgment by the second judgment device is recorded.