A first transistor and a second transistor form a current mirror circuit. A second current flowing in the second transistor is kept constant by a current control circuit. Therefore, a first current, to be output to a load, in the first transistor is kept at a constant value responsive to the second current in the second transistor. Since a drain voltage in the second transistor is controlled so as to become equal to a drain voltage in the first transistor, even if a voltage at an output terminal changes in response to a change in the impedance of the load, a ratio between the first current and the second current becomes substantially equal to a size ratio K between the first transistor and the second transistor. That is, the first current and second current precisely operate as a current mirror circuit.