An occupant detection system determines a seat occupancy condition including occupancy of a vehicle seat and a size of the occupant, that is, an adult or a child. The determined seat occupancy condition is sent to an air bag ECU as seat occupancy condition data used for air bag deployment. The system detects a velocity of the vehicle and compares it with a predetermined velocity threshold. When the velocity is equal to or lower than the threshold, the seat occupancy condition is determined and the determined condition is stored in a storage area as seat occupancy condition data. When the velocity is over the threshold, the stored seat occupancy condition date is send to the air bag ECU.