1. An X-ray CT apparatus that displays an image of the inside of an object based on projection data obtained by scanning the object, comprising: a generator that scans the object with each of X-rays of different energy levels and generates multiple projection data; a converter that converts the multiple projection data into multiple image data corresponding to multiple reference substances contained in the object based on a linear attenuation coefficient that varies according to the energy levels of the reference substances, and generates multiple new projection data corresponding to the reference substances according to the energy levels; and an identifier that identifies a target substance from an abundance ratio of each of the reference substances based on a fact that a pixel value at coordinates of each of the new projection data according to the energy levels is formed from a sum of values each obtained by multiplying the abundance ratio of each of the reference substances at the coordinates by the linear attenuation coefficient of the reference substance.