A television tuner comprises a connector 2 to which a television signal is input, a high frequency circuit 5 for frequency-converting the television signal into an intermediate frequency signal and outputting the intermediate frequency signal, and a power supply terminal 10 for supplying a power supply voltage to the high frequency circuit. And a power switch circuit 12 for supplying the power supply voltage to the connector is provided between the connector and the power supply terminal. Accordingly, by turning on/off the power switch circuit, the power supply voltage is supplied to the external circuit connected to the connector, and, when the power supply is stopped, the semiconductor device can be prevented from being damaged due to application of the over-voltage from the exterior to the connector.