An X-ray CT apparatus according to an embodiment includes an X-ray detector and a collimator unit. The X-ray detector detects X-rays that have passed through a subject. The collimator unit eliminates scattered radiation from X-rays that are incident on the X-ray detector. The collimator unit includes a plurality of collimator modules, a supporter, and a fixing unit. The plurality of collimator modules each includes a plurality of first collimator plates arranged in a grid along a channel direction and a slice direction that are orthogonal to each other. The supporter supports the collimator modules such that the collimator modules are aligned in a plurality of straight lines along the channel direction and in a plurality of straight lines along the slice direction. The fixing unit is provided to the supporter and fixes positions of the collimator modules in the channel direction and the slice direction.