A source controller unit for controlling an x-ray source is provided with a detection signal interface for receiving detection signals from detective pixels of an electronic cassette or an integrated value of the detection signals only, or an interface for receiving a radiation stopping signal only. The source controller unit receives other radiation signals than the radiation stopping signal, such as a radiation admitting signal, through a radiation signal interface. The source controller unit uses the detection signals, the integrated value thereof, or the radiation stopping signal as exposure control signals for stopping radiation from the x-ray source. Since the exposure control signals are received on the specific interface therefor, the source controller unit does not need signal sorting operation nor receive different kinds of signals at the same time, improving the speed of radiation stopping procedure.