1. An X-ray CT apparatus for exposing X-ray to a object to scan the object and reconstructing a CT image in the object based on obtained projection data, comprising: a transformation table acquiring unit configured to obtain a transformation table for transforming unspecified information representing a concentration of a contrast medium in a myocardium into an unspecified blood flow value image of the myocardium based on a CT image acquired in a concentration transition period defined to be a period from immediately after starting of a continuously injection of the contrast medium into the object until the contrast medium injected reaches to the myocardium, increases, and is to be in a state where it can be considered that the contrast medium is saturated in a constant value; a blood-flow information acquisition unit configured to obtain information representing the concentration of the contrast medium in the myocardium of the object based on a CT image acquired during a constant concentration period during which the concentration of the contrast medium in the myocardium can be considered to be constant; and a blood-flow image generating unit configured to generate a blood flow value image based on the information representing the concentration of the contrast medium in the myocardium in the myocardium according to the transformation table.