A simplified X-ray imaging apparatus is capable of computationally determining effective atomic numbers with small error factors even for light elements. In one embodiment, the X-ray imaging apparatus has an X-ray generation unit 101 (400) for generating X-rays and a detector 105 (405) for detecting X-rays transmitted through an object of examination 104 (403). A computing unit 106 (406) computationally determines a quantity of an X-ray phase attributable to the object of examination and an X-ray transmittance of the object of examination from data detected by the detector. The computing unit also computationally determines an effective atomic number of the object of examination from ρet determined from the quantity of the X-ray phase and μt determined from the X-ray transmittance.