This invention relates to a photoelectric switch and a photoelectric detector for detecting the presence or absence of an object by using a light, and more particularly to prevention of mutual interference in the case of using a plurality of photoelectric switches. For example, an LED in a light emitting unit intermittently emits a sensing light at the timing of an output pulse from a pulse generator. The resultant pulse is divided in frequency by a frequency divider and then delayed by a one-shot circuit. At the timing of an output pulse from the one-shot circuit, another LED emits a transmission light to a photoelectric switch at next stage. Therefore, the transmission light is always delayed from the sensing light. The transmission light from the preceding photoelectric switch of the same construction is received by a photodiode, and an output pulse is outputted from a pulse generator in synchronism with a synch timing signal produced from the light receiving unit. Thus, the sensing lights from the respective photoelectric switches are emitted at the timings shifted from one another through delays effected by the one-shot circuits.