An object of the invention is to provide an occupant protection system for a vehicle, in which an erroneous determination for a rear crash can be avoided. An output signal from an acceleration sensor for a front crash as well as an output signal from a back sonar sensor is also inputted to an electronic control unit (ECU), in addition to an output signal from a main rear crash sensor. Then, the ECU finally determines that a rear crash has occurred, only when the ECU respectively determines the rear crash from each of output signals from the main rear crash sensor, the front crash acceleration sensor and the back sonar sensor.