An encoder includes a light source. A booster circuit boosts a power supply voltage output by a battery, and outputs the boosted voltage. The voltage is applied to one end of a light-emitting diode. A driver circuit is inserted between another end of the light-emitting diode and a ground, the driver circuit controlling current flowing through the light-emitting diode. A voltage detector circuit detects a voltage between the light-emitting diode and the driver circuit. A control circuit causes the booster circuit to boost the power supply voltage when the voltage is lower than the power supply voltage; stops the boosting performed by the booster circuit when the voltage is equal to the power supply voltage; and controls the driver circuit such that, after the voltage reaches a predetermined value, current flows to the light-emitting diode.