A behavior detection apparatus detects a position of a head from an image; based on the position of the head, detects a motion of bending forward when a person to be detected sits up in a width direction of a bed in a bed area of the image and stands up from the sitting state; when a movement of the head is detected to be forward bending, sets, based on a movement path of the head, a stand-up detection line for determining bed leaving of the person between a lowest point of the forward bending and the position of the head when the person stands by the bed, the stand-up detection line being set in a position not overlapping with a forward bending detection line by which the motion of bending forward is detected; and detects a stand-up motion when the head passes the set stand-up detection line.