1. An object area detection device comprising: a first unit which detects an object area using an input image and background information not including an object; a second unit which, with respect to an area where a temporal change in the object area is small, detects an area (mass) having small spatial motion in a mass in an object unit within the area as a stationary area; a third unit which accumulates information of the stationary area as a plurality of pieces of background information arranged by time; and a fourth unit which compares the plurality of pieces of background information arranged by time with the object area and detects each object included in the object area.