An occupant detection system may detect a state of an occupant in a vehicle compartment. The occupant detection system may photograph the vehicle compartment and generate an image. The occupant detection system may change an attitude in order to change a position of an imaging region within the vehicle compartment. The occupant detection system may estimate the attitude based on information included in the image and control an operation based on the estimated attitude. The sensor may be provided by an infrared temperature sensor. The image may be provided by a thermal image showing a temperature distribution of the vehicle compartment. The occupant detection system may store a correspondence relationship between a position of a specific temperature boundary included in the thermal image and the attitude. The occupant detection system may estimate the attitude based on the correspondence relationship.