A first transistor may be provided on a current path of a photo transistor. The bias current path may include a bias switch and a first bias resistor connected in series and is provided in parallel with a main current path including the phototransistor. A second transistor may configure a current mirror circuit with the first transistor, and generates second current formed by multiplying the first current flowing in the first transistor by a predetermined factor. The second current is charged in the charging capacitor and converted into a voltage. The bias current path turns on prior to the start of light reception of the phototransistor and turns off after a predetermined period has elapsed from the start of light reception of the phototransistor.