Abstract: A semiconductor photoelectric detector comprising a detector unit and support device. The unit comprises light-emitting device, light-receiving device for receiving the light emitted by the light-emitting device, first and second wiring device electrically connected to the light-emitting device, third and fourth wiring device electrically connected to the light-receiving device, holder device holding the light-emitting and light-receiving device such that the optical axes of the light-emitting and -receiving device are aligned, and electrically insulating the first, second, third and fourth wiring device. The support device supports the detector unit such that the common optical axis of the light-emitting and -receiving device extends at right angle to that surface of the support device on which the detector unit is mounted, and has an electric current electrically connected to the first, second, third and fourth wiring device.