An occupant detection device includes: a radio wave sensor located above a seat surface of a seat that is disposed in a vehicle cabin of a vehicle, in a vertical direction of the vehicle, and including a transmission unit configured to transmit a transmission wave to the vehicle cabin and a reception unit configured to receive a reflected wave generated by the transmission wave being reflected by an occupant on the seat; a creation unit configured to create, based on the reflected wave, three-dimensional map information in the vehicle cabin in which a position of a reflection point of the reflected wave is represented by three-dimensional coordinates; and a calculation unit configured to calculate, based on the three-dimensional map information, backbone information that is information on a backbone of the occupant on the seat.