Provided are a radiographic image detection device which is capable of reducing the power consumed by a signal processing circuit during an emission start detection operation for detecting the start of radiation emission, and a method for operating the same. A control unit (54) of an electronic cassette (16) has the function of switching the state of supply of power to an ADC (77) between an operating state that corresponds to a first state for supplying first power and a non-operating state that corresponds to a second state for supplying second power, the power of which per unit of time is lower than that of the first power. The control unit (54) reduces the number of ADCs (77) in the operating state per unit of time T when an AED operation for detecting the start of X-ray emissions is being performed in comparison to when an image retrieval operation for outputting an X-ray image for diagnosis is being performed.