A determination device (2, 2c) for outputting a signal indicating intrusion detection when a distance to an object is within a predetermined range, the determination device (2, 2c) comprising: a plurality of acquisition parts (221, 223) each acquiring a signal outputted by its optical distance measurement sensor (21a, 21al, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) that is included in a sensor set (25, 26, 27) and Z wherein each of the optical distance measurement sensors of the plurality of acquisition parts has observation areas (R1, R2) overlapping each other; a plurality of abnormality determination parts each determining that an abnormality has occurred in at least one of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) when distances respectively indicated by the signals received by the acquisition parts (221, 223) deviate from each other by a predetermined length or more; and an output part (24a, 24b) outputting a signal indicating the abnormality when at least one of the abnormality determination parts determines that the abnormality has occurred; the determination device (2, 2c) being characterized in that the optical distance measurement sensor (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) comprises a multi-pixel light receiving part comprising a plurality of light receiving elements, the determination device (2, 2c) further comprises an extraction part extracting light receiving elements corresponding to an overlapping observation area (R3, R100) of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3), among the light receiving elements of the light receiving part, as light receiving elements in a valid range, wherein the extraction part extracts the light receiving elements in the valid range according to a distance between the object in the observation area and the optical distance measurement sensor (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) and each of the abnormality determination parts determines whether the abnormality has occurred in at least one of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3,21b, 21b1, 21b2, 21b3) by using signals outputted by the light receiving elements in the valid range. a plurality of acquisition parts (221, 223) each acquiring a signal outputted by its optical distance measurement sensor (21a, 21al, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) that is included in a sensor set (25, 26, 27) and Z wherein each of the optical distance measurement sensors of the plurality of acquisition parts has observation areas (R1, R2) overlapping each other; a plurality of abnormality determination parts each determining that an abnormality has occurred in at least one of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) when distances respectively indicated by the signals received by the acquisition parts (221, 223) deviate from each other by a predetermined length or more; and an output part (24a, 24b) outputting a signal indicating the abnormality when at least one of the abnormality determination parts determines that the abnormality has occurred; the determination device (2, 2c) being characterized in that the optical distance measurement sensor (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) comprises a multi-pixel light receiving part comprising a plurality of light receiving elements, the determination device (2, 2c) further comprises an extraction part extracting light receiving elements corresponding to an overlapping observation area (R3, R100) of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3), among the light receiving elements of the light receiving part, as light receiving elements in a valid range, wherein the extraction part extracts the light receiving elements in the valid range according to a distance between the object in the observation area and the optical distance measurement sensor (21a, 21a1, 21a2, 21a3, 21b, 21b1, 21b2, 21b3) and each of the abnormality determination parts determines whether the abnormality has occurred in at least one of the optical distance measurement sensors (21a, 21a1, 21a2, 21a3,21b, 21b1, 21b2, 21b3) by using signals outputted by the light receiving elements in the valid range.