A dimmer, according to an embodiment, includes a reference voltage circuit configured to output a reference voltage at a constant voltage when a power source voltage is higher than or equal to a predetermined threshold value, and output the reference voltage at a voltage proportional to the power source voltage when the power source voltage is lower than the predetermined threshold value. The dimmer further includes a bias current circuit; a control current circuit; an analog-to-digital converter; an output signal generation circuit; a first constant-current source; and a switch connected to the first constant-current source.