PURPOSE: To decrease the delay time at turn-off of a phototransistor (TR), by connecting a voltage clamping element between the collector and ground of the photo TR.

CONSTITUTION: When an input signal is applied between signal input terminals 2 and 3, a current flows to an LED4 being a light emitting element of a photo- coupler PC and the LED4 is lighted. Then, the photo TR5 being a photo detecting element receiving the light is turned on and the input signal is transmitted to a TRQ1 at the post stage. Diodes D1, D2 are connected between the collector and ground of the photo TR5 and the collector potential of the photo TR is clamped with the forward voltage of the diodes, thereby preventing an excessive charge to a junction capacitance of the photo TR and decreasing the turn-off time of the photo TR.