To make it possible to use a combination jack while minimizing a burden on the port of a microcomputer.

The connection of a stereo headphone plug and a monaural microphone plug is made possible using a combination jack and a circuit is switched in accordance with a connection plug. A control means (controller 1) detects the connection of the plug to the combination jack from a potential state at a detection port P2. A discrimination signal is then applied and a voltage state between a second signal terminal T3 and a ground terminal is detected at the detection port P2. Consequently, the detection of plug connection and the detection of the type of a connected plug (whether the stereo headphone plug or the monaural microphone plug) are performed only at one detection port P2.