The X-ray CT apparatus includes an X-ray tube, an X-ray detector, a rotation unit, a data generation unit, an image generation unit and a setting unit. The data generation unit generates data used in reconstruction processing of a CT image on a basis of an output of the X-ray detector. The image generation unit performs reconstruction processing of the CT image, the reconstruction processing using the data and a parameter value relating to at least one of an FDD and an FCD. The setting unit sets the parameter value used in the reconstruction processing using the data generated when the X-ray is emitted to the object, the parameter value being set based on CT values of a plurality of CT images obtained by a plurality of parameter values used in the reconstruction processing using the data generated when the X-ray is emitted to a phantom.
