In a disconnection detecting mode, first and second MOSFETs are turned off, a transistor is turned on, and charges of a capacitor are initialized. After that, third and fourth MOSFETs are alternately turned on/off by drive signals and having a complementary relation. When an antenna is connected normally, a pulse AC signal is transmitted to an output terminal via the antenna, and current flows from the output terminal into the capacitor via a resistor, a diode, and a resistor. When a voltage across terminals becomes equal to or higher than a determination reference voltage, a disconnection detection signal is changed to the H level (a connection state).