A passive safety system has a passive safety device mounted on a vehicle, and a passive safety control means having an acceleration sensor detecting acceleration caused by an impact at the time of collision. The passive safety control means receives input an impact acceleration detection signal from the acceleration sensor and operates controllably the passive safety device. Further, the passive safety control means computes a physical quantity based on the impact acceleration detection signal inputted from the acceleration sensor, sets maximum and minimum reference values of the physical quantity in normal driving, performs a computation with respect to addition of a present acceleration inputted from the acceleration sensor to an integrated acceleration value at this point in time when the physical quantity crosses a range defined between the maximum and minimum reference values, and performs a computation with respect to a reset process of the integrated acceleration value when the physical quantity remains within the range defined between the maximum and minimum reference values.