A flat panel detector (FPD) includes an imaging panel having pixels arranged in a matrix, a gate driver for turning thin film transistors (TFTs) of the pixels ON and OFF, a radiation detecting section for detecting the start of x-ray radiation from the x-ray source, and a controller. The controller controls the gate driver to turn the TFTs ON periodically to reset dark charges of the pixels. Before starting a charge accumulating operation for accumulating signal charges for imaging, the controller controls the gate driver to turn the TFTs OFF so that the radiation detecting section may detect the start of x-ray radiation on the basis of charge leaks from the pixels. When the start of x-ray radiation is detected, the controller starts the charge accumulating operation while keeping the TFTs in the OFF condition. Thereafter, the TFTs are turned ON to read out the accumulated signal charges.