An occupant protection system is provided. The occupant protection system includes a sensor for detecting an impact at a time of collision, a determination device for determining the collision based on a detection signal of the sensor, and a an occupant state detection device for detecting whether an occupant's posture is maintainable. When the occupant's posture is maintainable after a primary-collision, the determination device determines a secondary-collision by comparing the detection signal of the sensor with a first threshold. When the occupant's posture is not maintainable after the primary collision, the determination device determines the secondary-collision by comparing the detection signal of the sensor with a second threshold smaller than the first threshold.