To provide a rectifier such that the voltage between its AC input terminal and DC output terminal when the rectifier is turned on is lower than the forward voltage which varies in accordance with the physical properties of a diode.

A drain electrode and a source electrode of a field-effect transistor 1 are respectively used as the AC input terminal and DC output terminal of a rectifier, and the gate electrode of the transistor 1 is connected to the output of an operational amplifier 2. In addition, the inputs of the amplifier 2 are respectively connected to the AC input terminal and the DC output terminal of the rectifier. The amplifier 2 detects the potential difference between the AC input terminal and DC output terminal, and when an input voltage is impressed in the reverse direction, amplifier turns off the transistor 1. When the input voltage is impressed in the forward direction, the amplifier 2 causes the transistor 1 to be energized.