A simplified X-ray imaging apparatus is capable of computationally determining effective atomic numbers with small error factors even for light elements. 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 [Rho]et determined from the quantity of the X-ray phase and [mu]t determined from the X-ray transmittance.