A flat panel detector has pixels for obtaining image signals and detective pixels for detecting the amount of incident x-rays. A signal processing circuit is of a pipeline-type, wherein first and second buffer memories are connected to the output of an A/D converter. In a dose detecting operation, the signal processing circuit repeats primary cycles alternately with secondary cycles of a shorter length than the primary cycles. In the primary cycle, a dose detection signal based on electric charges from the detective pixels is input in the first buffer memory and, simultaneously, a dummy signal is output from the second buffer memory. In secondary cycle, the dose detection signal is output from the first buffer memory and, simultaneously, a second dummy signal is input in the second buffer memory. On the basis of the dose detection signals, a start-of-radiation detector detects the start of x-ray radiation.