PURPOSE: To eliminate fluctuation of a pulse width between the input and output and at the same time to assure the electrical insulation, by forming an input circuit with two photocouplers.

CONSTITUTION: A signal repeater is provided with an input circuit 3 containing light emitting diodes 1a and 2a of two photocouplers 1 and 2 connected adversely in parallel and receives the input of the AC pulse signal, a zero cross pulse generating circuit 4 which produces the zero cross pulse when the outputs of photo TR1b and TR2b of the circuit 3 are set at the same level, signal reproducing circuit 5 consisting of a flip-flop, and a driving circuit 6 which delivers an AC pulse signal. For such repeater, fluctuation of the pulse width between the input and output signals which is caused by the forward threshold voltage of the light emitting diode of the photocoupler is eliminated by forming an input circuit with two photocouplers.