A calibration method for an X-ray measuring device includes mounting a calibration tool on a rotating table, identifying centroid positions from an output of an X-ray image detector, calculating projection transformation matrixes from the centroid positions and known relative positional intervals, repeating to identify the centroid positions from the output of the X-ray image detector and to calculate the projection transformation matrixes from the centroid positions and known relative positional intervals while the rotating table is rotated twice or more by a predetermined angle, and calculating a rotation center position of the rotating table on the basis of the projection transformation matrixes. The calibration method thereby allows easy calculation of the rotation center position of the rotating table on which an object to be measured is mounted in a rotatable manner, with the simple process.