By using the co-charging system of this invention, a 12V battery such as a lead storage battery is not required, so that depreciation cost can be suppressed. In order to realize above, power should be always supplied to the power supply node of the control circuit by following. (1) Connect the diode so that current flows from the power supply supplied from the solar cell, the power supply supplied from the AC adapter, the power supply supplied from the mobile battery, to the power supply node of the control circuit. (2) Connect a large-capacity capacitor such as an electric double layer capacitor to the power supply of the control circuit. (3) The inverter has an nMOS field-effect transistor+resistor configuration, and the inverter has an nMOS field-effect transistor+resistor+pMOS field-effect transistor configuration.