An optical safety sensor, comprising: a group consisting of a plurality of light projectors/receivers (20A, 20B), which is provided with light projecting portions projecting light to a monitoring area and light receiving portions receiving reflected light from the monitoring area; and a plurality of distance measurement portions, which measures distances to a subject within the monitoring area using time required from the light projecting to the light receiving; a plurality of detection portions, which detects, based on measurement results obtained by the distance measurement portions, a malfunction of any one of the light projectors/receivers (20A, 20B) occurring in any one of the plurality of light projectors/receivers (20A, 20B), wherein one of the light projectors/receivers (20A, 20B) forms one set with a corresponding one of the distance measurement portions and a corresponding one of the detection portions; and each of the light receiving portions provided in the plurality of light projectors/receivers (20A, 20B) receives the reflected light caused by the light projected from the light projecting portions of all the plurality of light projectors/receivers (20A, 20B) wherein when a measured distance based on the time from the light projecting by the light projecting portion of a first light projector/receiver, which is one light projector/receiver of the plurality of light projectors/receivers (20A, 20B), to the light receiving by the light receiving portion of the first light projector/receiver does not match with a measured distance based on the time from the light projecting by the light projecting portion of a second light projector/receiver, which is different from the first light projector/receiver, to the light receiving by the light receiving portion of the second light projector/receiver, the detection portions determine that the malfunction of any one of the light projectors/receivers (20A, 20B)occurs in any one of the first light projector/receiver and the second light projector/receiver, wherein when the measured distance based on the time from the light projecting by the light projecting portion of the first light projector/receiver to the light receiving by the light receiving portion of the first light projector/receiver does not match with the measured distance based on the time from the light projecting by the light projecting portion of the second light projector/receiver to the light receiving by the light receiving portion of the first light projector/receiver, the detection portions determine that the malfunction of any one of the light projectors/receivers (20A, 20B)occurs in any one of the light projecting portion of the first light projector/receiver and the light projecting portion of the second light projector/receiver. a group consisting of a plurality of light projectors/receivers (20A, 20B), which is provided with light projecting portions projecting light to a monitoring area and light receiving portions receiving reflected light from the monitoring area; and a plurality of distance measurement portions, which measures distances to a subject within the monitoring area using time required from the light projecting to the light receiving; a plurality of detection portions, which detects, based on measurement results obtained by the distance measurement portions, a malfunction of any one of the light projectors/receivers (20A, 20B) occurring in any one of the plurality of light projectors/receivers (20A, 20B), wherein one of the light projectors/receivers (20A, 20B) forms one set with a corresponding one of the distance measurement portions and a corresponding one of the detection portions; and each of the light receiving portions provided in the plurality of light projectors/receivers (20A, 20B) receives the reflected light caused by the light projected from the light projecting portions of all the plurality of light projectors/receivers (20A, 20B) wherein when a measured distance based on the time from the light projecting by the light projecting portion of a first light projector/receiver, which is one light projector/receiver of the plurality of light projectors/receivers (20A, 20B), to the light receiving by the light receiving portion of the first light projector/receiver does not match with a measured distance based on the time from the light projecting by the light projecting portion of a second light projector/receiver, which is different from the first light projector/receiver, to the light receiving by the light receiving portion of the second light projector/receiver, the detection portions determine that the malfunction of any one of the light projectors/receivers (20A, 20B)occurs in any one of the first light projector/receiver and the second light projector/receiver, wherein when the measured distance based on the time from the light projecting by the light projecting portion of the first light projector/receiver to the light receiving by the light receiving portion of the first light projector/receiver does not match with the measured distance based on the time from the light projecting by the light projecting portion of the second light projector/receiver to the light receiving by the light receiving portion of the first light projector/receiver, the detection portions determine that the malfunction of any one of the light projectors/receivers (20A, 20B)occurs in any one of the light projecting portion of the first light projector/receiver and the light projecting portion of the second light projector/receiver.