When using a photocoupler with the base terminal (TLP731, for example), if the base terminal is left open, it works as an antenna and becomes likely to receive external noise easily. Especially, if a pulsed high voltage is applied between the input and output devices, induction noise received by the lead terminal of the base terminal and noise that flows by passing along the capacitance (input to output) are added, making the photocoupler operate as if there were input signals. In such a case, malfunction can be prevented by inserting capacitor CBE in the output transistor between the base and the emitter to absorb transient noise on the base terminal with CBE. However, this makes the output signal response rate slow. Care should be taken.