To enable exact judgement of the quickly decelerated state of a vehicle even when acceleration at the time of the quick deceleration of the vehicle exceeds the detection range of an acceleration sensor.
Acceleration G detected by an acceleration sensor is read in, and when the acceleration G reaches the upper limit value GMAX of the detection range of the acceleration sensor, an increment is carried out in a counter D(step 120-122, 128). After that, the counted value of the counter D is compared (step 130, 134), and a constant A is set up to any of set values α, β, γ according to elapsed time after exceeding the upper limit value GMAX (step 132, 136, 138). After that, an acceleration GX for judging the quickly decelerated state of the vehicle from the constant A and the counted value of the counter D is computed (step 126).
YOSHIDA YUTAKA