To charge a secondary battery by simple constitution in a key device having an illumination function.

A coil 116 for a key section 102 receives a high-frequency signal from a coil 104 for a main body instrument 103, and a charging circuit 115 charges a secondary battery 114 by the output signal of the coil 116. An LED 117 is flashed during the charging period, and the flashing is input to a control circuit 106 through a photo-diode 107, an amplifier circuit 108 and a waveform shaping circuit 109. The control circuit 106 controls a transmitter-receiver circuit 105 so that the high-frequency signal is transmitted from the coil 104 during a time when the LED 117 is flashed. When a user operates a control switch 111, on the other hand, the LED 117 emits light by the power of the secondary battery 114.