An X-ray computer tomography apparatus includes at least one X-ray tube generating X-rays, first detector elements (energy integrating type) and second detector elements (photon counting type) detecting an intensity and spectrum of the X-rays transmitted through the object respectively, at least one data acquisition circuit acquiring first projection data and second projection data smaller in data amount than the first projection data detected by the first and second detector elements respectively, an arithmetic circuit computing a minimum value of a cost function based on the first and second projection data by executing an iterative reconstruction algorithm, and reconstruction circuit reconstructing an image of the object based on the first and second projection data, which correspond to the minimum value of the cost function.
