A stacked type of radiation detector and a calibration method that enables the radiation detector to correct variations in sensor-specific output easily and within a short time, without using a water-phantom dose detector. The radiation detector is equipped with a sensor section including a plurality of sensors arranged in layers in a traveling direction of a particle beam. A dummy absorber has water-equivalent thickness equal to an average water-equivalent thickness of the sensors. A signal-processing unit calculates sensor-specific calibration coefficients using a measurement result obtained during irradiation of the radiation detector with the radiation when electrical signals developed in each sensor are measured, and a measurement result obtained during irradiation of the radiation detector with the radiation when the sensor section is moved in the traveling direction of the radiation, then the dummy absorber is set in place, and electrical signals developed in each sensor are measured.
