An electronic cassette comprises a main battery which is detachably attached to a battery loading unit, and a sub-battery which supplies electricity to a bias power circuit and so on in substitution for the main battery. A power source selector changes the power source to the sub-battery from the main battery when it is judged that a replacement operation of the main battery is started. Since supply of the electricity to the bias power circuit is continued, and the bias voltage continues being applied to a photoelectric converter without a break, there is no need to perform a photoelectric conversion stabilizing process and an offset correction image detecting process after turning on of the main power. Therefore, a start-up time TR2 is largely shortened from a start-up time TR1.