A collision sensor provided with an accelerometer which outputs acceleration signals; first arithmetic means which calculates the magnitude of the physical quantity of the first wave of the acceleration signal from the time at which an acceleration is made; second arithmetic means which calculates the deceleration corresponding value corresponding to the degree of deceleration based on the acceleration signal; comparing means which compares the deceleration corresponding value calculated by the second arithmetic means with a prescribed threshold value, and, when the deceleration corresponding value is larger than the threshold value, judges that a collision occurs and outputs a start signal; and a triggering circuit which starts such an occupant protective device as an air bag based on the start signal. The sensor is further provided with a correcting means which corrects either one or both of (a) the deceleration corresponding value and (b) the threshold value in accordance with the magnitude of the physical quantity of the first wave calculated by the first arithmetic means.