The invention relates to a radiation image detecting device and a control method thereof. When all TFTs are turned on, an electric signal is compared with a first threshold value. If the electric signal is equal to or more than the first threshold value, a first judgment unit judges that X-ray irradiation has been started. A second judgment unit compares second and third threshold values with a first-order differentiation value of an electric signal that is outputted in a state of turning off all the TFTs. If the first-order differentiation value is within or out of a range defined by the second and third threshold values throughout a verification period, the second judgment unit verifies that the judgment of the first judgment unit is correct. When the judgment of the first judgment unit is verified to be correct, the TFTs are kept turned off, and an FPD continuously carries out charge accumulation operation for capturing an X-ray image.