A radiation detector includes a sensor panel, a scintillator panel, a reflective layer, and a radiation irradiation detecting photodetector laminated in this order from a side of a radiation receiving surface. Radiation transmitted through a patient's body enters the scintillator panel through the sensor panel, and is converted into light. The converted light propagates through columnar crystals in the scintillator panel with total internal reflection. Apart of the light reaches the sensor panel, while the remains reach the reflective layer. The light reaching the sensor panel is detected by photoelectric converters. Out of the light reaching the reflective layer, a short wavelength component with a relatively high refractive index is specularly reflected to the sensor panel. A long wavelength component with a relatively low refractive index is transmitted through the reflective layer, and enters the radiation irradiation detecting photodetector, which detects a start of radiation irradiation.