A bias current supplying circuit having: a differential amplifier for amplifying a difference between potentials at first and second nodes and outputting the amplified difference voltage from a third node, the first node being connected to an output terminal of a bias voltage source for outputting a bias voltage, and the second node being connected to an output terminal from which a bias current is supplied to an external circuit; a negative feedback loop circuit for changing the potential at the second node in accordance with the potential at the third node of the differential amplifier; and a potential dropping circuit connected to the second node for dropping the potential at the second node in response to an external input of a control signal and intercepting a supply of the bias current to the external circuit.