In order to improve linearity of an input signal and an output signal without using an amplifier, a current Iin is divided into a current I1 and a current I2. When the current I1 passes through a light emitting diode D1, the light-emitting diode D1 emits light, and a current path of a phototransistor Q1 conducts. At that time, the current Iout passes through the light emitting diode D2, and the light-emitting diode D2 emits light. When the light emitting diode D2 emits light, the current path of a phototransistor Q2 conducts, and the current I2 flows. For example, if the current Iout is too large, then the emission intensity of the light-emitting diode D2 becomes too great. At that time, a large current I2 passes through the phototransistor Q2. For that reason, the current I1 flowing in the light emitting diode D1 decreases. By this means, the light emission intensity of the light-emitting diode D1 becomes smaller, so the current Iout decreases. In this way, the photocoupler P2 functions as a negative feedback circuit, so the linearity of current Iin and the current Iout increases.