A collision detecting device providing the accurate and early determination of a collision with a vehicle or the like and a passive safety system using this collision detecting device. A low-rigid portion and a crash box are arranged ahead of the high-rigid portion in a front portion of a frame. The front end of a rod is fixed to a bumper. The rod is inserted into a magnetic detector disposed on the high-rigid portion. In the rod, a number of magnets are aligned in the longitudinal direction of the rod such that their N poles and S poles are alternatively arranged. As a vehicle collides, the crash box is first deformed so that the rod moves backward, whereby a collision can be detected before large acceleration is generated.