Disclosed is an X-ray diffraction apparatus that irradiates a sample with X-ray emitted from an X-ray source by resting the X-ray using a divergence slit and detects diffracted X-ray generated from the sample using an X-ray detector. The divergence angle of the divergence slit is a fixed value, and the divergence slit is a slit that restricts the X-ray irradiation width in the sample width direction. The sample is arranged in a longitudinally-elongated manner in which its sample width is smaller than a standard sample width and its sample height is the same as a standard sample height. X-ray intensity calculated based on an output of the X-ray detector is compensated based on an effective divergence angle calculated based on the sample width to thereby obtain true X-ray intensity.