A reference-height calculating unit calculates an original height of a distortion detecting mirror when a distortion detecting mirror and a reference mirror are set in parallel to each other. A height measuring unit measures mirror heights when the distortion detecting mirror is tilted in directions of an X axis and a Y axis. A contour-line calculating unit calculates a pair of contour lines based on the mirror heights. An intersection calculating unit calculates an intersection of the contour lines. A storing unit stores coordinates to which height of predetermined coordinates are moved by distortion in a correction table. A distortion correcting unit corrects the distortion based on the correction table.
