Before a radiation image is captured, a rate of change of an output signal from an integrator, during a given period of time after the integrator has been reset and until a radiation start signal is supplied, is calculated. An offset voltage signal at a desired time is calculated using the rate of change, and is supplied to a voltage correcting circuit. An output signal from the integrator after a radiation X has started being applied to a subject is corrected based on the calculated offset voltage signal. The corrected output signal from the integrator is supplied to an X-ray tube controller for controlling application of the radiation X to the subject.