A device housing a battery (50) includes a receiving coil (51), and a charging pad (10) includes a transmitting coil (11) that magnetically couples with, and supplies charging power to the receiving coil. The device further includes a modulator circuit (61) that changes the impedance of the receiving coil according to internal battery data. The charging pad further includes a detection circuit (17) that detects receiving coil impedance changes to detect the battery data. The modulator circuit has a load circuit (62) connected in parallel with the receiving coil and has a series-connected switching device (64) and impedance modulating capacitor (63), and a control circuit (65) that switches the load circuit switching device ON and OFF according to the battery data. The modulator circuit switches the switching device 64 ON and OFF to transmit battery data to the charging pad.