To provide an X-ray CT apparatus capable of correcting a difference of scattered radiation quantities contained in the projection data even in a case where a plurality of detection elements are disposed between collimator plates. The X-ray CT apparatus includes: an X-ray irradiation section for X-ray radiation； an X-ray detector including a plurality of detection elements for detecting the X-ray； a plurality of collimator plates disposed between the X-ray irradiation section and the X-ray detector so as to reduce scattered radiation； a reconstruction portion for reconstructing a tomographic image by using projection data generated based on an output from the X-ray detector； and a correction portion for correcting the projection data by using different correction functions according to the positions of the plural detection elements disposed between the collimator plates.
