An activation apparatus for an occupant protection system includes acceleration sensors which are disposed on the respective right and left sides of a vehicle with respect to the center of the vehicle on the vehicle and each of which is provided for detecting the acceleration from the side opposite to the side where the sensor is disposed, and includes a determiner that determines whether or not an occupant protection system is to be activated by comparing the physical quantity computed based on the value detected and outputted by the acceleration sensor with a threshold for activating the occupant protection system.