Based on counts detected by a photon counting detector, a characteristic of X-ray attenuation amounts μt is acquired for each X-ray energy bin. This characteristic is defined by a plurality of mutually different known thicknesses t and linear attenuation coefficients in the X-ray transmission direction. This substance is composed of a material which is included in an object and which is the same in type as the object or which can be regarded as being similar to the object in terms of the effective atomic number. Correcting data for replacing the characteristic of the X-ray attenuation amounts μt by a linear target characteristic are calculated. The linear target characteristic is set to pass through the origin of a two-dimensional coordinate having a lateral axis assigned to thicknesses t and a longitudinal axis assigned to the X-ray attenuation amounts μt. The correcting data are calculated for each X-ray energy bin.
