A Method for X-ray wavelength measurement and an X-ray wavelength measurement apparatus capable of determining absolute wavelength easily and carrying out wavelength measurement having high precision with a simple structure are provided. The present invention is a Method for X-ray wavelength measurement carried out by using a channel-cut crystal for wavelength measurement (20) in which two opposing cut planes are formed and the lattice constant of which is known, and the method diffracts X-ray in respective arrangements (−, +) and (+, −) of the channel-cut crystal for wavelength measurement (20), to determine the absolute wavelength of the X-ray from the difference between crystal rotation angles in respective arrangements. This makes the alignment simpler, and, when only a channel-cut crystal suitable for measurement can be prepared, X-ray wavelength measurement can be carried out easily and with high precision.