Abstract: A first circuit having a first coil electrically charges a second circuit having a second coil through electromagnetic coupling of the two coils. When data signals are to be transferred between the first and second circuits, signal transfer is started only after the second circuit has been charged for a predetermined period of time. The position relationship between the coils is also detected, and a charging/transfer selector changes a duty ratio between charge transfer and data transfer in accordance with the detected result. The charge is transferred in an intermittent manner, and the charging rate is adjusted according to the difference between the voltage of a secondary battery observed during a charging phase and the voltage of the secondary battery observed a certain time after interruption of the charging phase, or vice versa.