To provide a radiation image detecting device providing high responsivity and high precision of an emission start judgment, an electronic cassette has a panel unit and a control unit. The panel unit has a two-dimensional array of normal pixels for accumulating signal charge upon receiving X-rays and detection pixels for detecting the X-rays. A signal processing circuit periodically samples a dose signal, corresponding to an X-ray dose per unit of time, from the detection pixels. An emission start judgment unit performs based on the dose signals of the detection pixels a first judgment process for judging whether X-ray emission has been started, and a second judgment process for judging whether a result of the first judgment process is correct. The control unit sets a second sampling cycle SP2 used in the second judgment process longer than a first sampling cycle SP1 used in the first sampling process.