A two-dimensional position-sensitive X-ray detector is used for the precision measurement of lattice constants so that a plurality of X-ray rocking curves can be measured at the same time for the respective points on a sample and an area map, on the sample, of the lattice constants can be obtained in a short time. X-rays from an X-ray source pass through the first slit and are then incident on a crystal collimator. X-rays reflected by the crystal collimator are incident on the sample. X-rays diffracted at the sample are detected by the two-dimensional position-sensitive X-ray detector. The diffracted X-rays from the respective points on the sample are detected separately at respective points on the X-ray detector. X-ray intensities which are detected at respective points on the detecting surface of the X-ray detector are recorded, at the same time, at every rotation angle with a predetermined pitch of angle during sample rotation, so that a plurality of rocking curves for said respective points of the sample can be obtained at the same time.