The numbers of Panasonic ST4 sensor heads and controllers can be freely adjusted to meet the heights and the required numbers of the protection area. The beam interruption indicator is incorporated in both the emitter and receiver. This indicator can be used not only for operation confirmation but also for beam axis alignment. Moreover, the stability indicator indicates if the incident light intensity exceeds 150 % in stable operation. Light received condition of the sensor heads in series connection can be confirmed by the high-functional controller ST4-C12EX. In addition, any abnormal sensors during lockout can be identified. The Type 4 long sensing range type has a compact size that is equivalent to those of general-purpose photoelectric sensors. Having the same mounting pitch as those of general-purpose photoelectric sensors makes model switchovers easy. Conforming to protection structure IP67, the sensor heads can be used safely even at lines where water splashes. The emission amount adjuster can be used to reduce the emission to control any interference to the surrounding sensors. A single unit can be used for PNP / NPN output switching, reducing the number of parts that need to be registered.