A flatness calibration method comprising: moving sequentially one rectangular parallelepiped-shaped bar mirror, which includes a plane having a scheduled measurement range with a size corresponding to a part of a measurement range in XY-directions of a Z-axis reference mirror of a shape measurement device and is formed in a bar shape, at each of positions in an X-direction, and after the bar mirror is rotated by 90° in an XY-plane at each of positions in a Y-direction within the XY-plane, sequentially measuring flatness at each of the positions of the plane of the bar mirror in line shapes, and obtaining measurement data in a line shape in the X-direction and measurement data in a line shape in the Y-direction, by a measurement data obtaining device; moving the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction in a Z-direction, by a data moving unit so that the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction approach each other in the Z-direction, at an intersection position of a line in the X-direction including the obtained measurement data in the line shape in the X-direction and a line in the Y-direction including the obtained measurement data in the line shape in the Y-direction; synthesizing the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction, which are moved by the data moving unit, and generating a synthesis plane, by a synthesis plane calculator; obtaining a twist correction amount in the Z-direction at a lattice point of the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction, when generating the synthesis plane, and determining a twist correction factor so as to minimize the obtained twist correction amount, by a twist correction factor determiner; correcting the twist correction amount of the synthesis plane by using the twist correction factor determined by the twist correction factor determiner, by a corrector; and calibrating flatness of the measurement range of the Z-axis reference mirror in the XY-directions by using the twist correction amount corrected by the corrector. moving sequentially one rectangular parallelepiped-shaped bar mirror, which includes a plane having a scheduled measurement range with a size corresponding to a part of a measurement range in XY-directions of a Z-axis reference mirror of a shape measurement device and is formed in a bar shape, at each of positions in an X-direction, and after the bar mirror is rotated by 90° in an XY-plane at each of positions in a Y-direction within the XY-plane, sequentially measuring flatness at each of the positions of the plane of the bar mirror in line shapes, and obtaining measurement data in a line shape in the X-direction and measurement data in a line shape in the Y-direction, by a measurement data obtaining device; moving the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction in a Z-direction, by a data moving unit so that the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction approach each other in the Z-direction, at an intersection position of a line in the X-direction including the obtained measurement data in the line shape in the X-direction and a line in the Y-direction including the obtained measurement data in the line shape in the Y-direction; synthesizing the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction, which are moved by the data moving unit, and generating a synthesis plane, by a synthesis plane calculator; obtaining a twist correction amount in the Z-direction at a lattice point of the measurement data in the line shape in the X-direction and the measurement data in the line shape in the Y-direction, when generating the synthesis plane, and determining a twist correction factor so as to minimize the obtained twist correction amount, by a twist correction factor determiner; correcting the twist correction amount of the synthesis plane by using the twist correction factor determined by the twist correction factor determiner, by a corrector; and calibrating flatness of the measurement range of the Z-axis reference mirror in the XY-directions by using the twist correction amount corrected by the corrector.