An occupant detection device of an embodiment includes, for example: an acquirer that acquires captured image data output from an image capturer capturing a seat and distance data indicating a distance to an occupant present in a direction of the seat; a seat specifier that specifies a position of a surface of the seat; a trunk surface specifier that specifies a position of a surface representing a trunk of the occupant based on the captured image data and the distance data; and a calculator that calculates a volume of the trunk of the occupant present between the seat and the image capturer based on the position of the surface representing the trunk and the position of the surface of the seat.