An optical divider (7), (8) is disposed, in front of at least one of the transmitter and receiver elements (3), (4), therebetween, and at least one of a transmission and reception side, and has a plurality of optical dividing pieces (7S), (8S) each having an optical deflection surface (7a), (7b), (8a), (8b) on which an angle of a detection ray is varied to a plurality of angles for dividing the detection ray toward a plurality of division areas. The optical deflection surface (7a), (7b), (8a), (8b) on at least one of the transmission and reception side is set to have a given angle, to assign at least one of the detection ray from the transmitter elements (3) and toward the receiver elements (4), to a given division area, such that a combination of the transmitter and receiver elements (3), (4) forming one of the division areas is different from a combination of the transmitter and receiver elements (3), (4) forming another one of the division areas.