To accurately detect the presence or absence of a signal. A signal detector includes an input-signal amplifying circuit, a reference-signal amplifying circuit, and a comparator. In the signal detector, the input-signal amplifying circuit amplifies an input signal with a predetermined gain. The reference-signal amplifying circuit amplifies a reference signal at a constant signal-level with a gain that substantially matches the predetermined gain. The comparator compares a signal level of the amplified input signal with a signal level of the amplified reference signal, and outputs the comparison result as a detection signal.