An occupant state detection system includes an occupant monitoring apparatus, a determination apparatus, and an occupant state detection apparatus. The occupant state detection apparatus includes one or more processors and one or more memories. The one or more memories store data including traveling-state comparative data and stopped-state comparative data. The one or more processors cooperate with one or more programs included in the one or more memories to receive occupant monitoring data from the occupant monitoring apparatus and determination data from the determination apparatus. On the basis of the determination data, the one or more processors compare the occupant monitoring data with the traveling-state comparative data on the condition that the vehicle is in the traveling state, and compare the occupant monitoring data with the stopped-state comparative data on the condition that the vehicle is in the stopped state, to detect a state of the occupant in the vehicle.