A radiation image detector including: an internal power supply section, which is capable of being charged, to supply power to each section; an external power supply terminal to receive power from an external power source to allow the internal power supply section to be charged; and a power supply control section to control supply of the power to the internal power supply section, the power being received from the external power source via the external power supply terminal, wherein when detecting that the external power supply terminal receives power from the external power source, the power supply control section judges an operating state of each section, and controls supply of the power to the internal power supply section according to the operating state of each section, the power being received from the external power source via the external power supply terminal.