To represent undulation faithfully by operating an undulation inclination angle at the intersection of diagonals of four lattice points, as a vector being defined by a magnitude 7 and an orientation angle θ, from a profile data of a reference plane measured as a deviation value at a sampling point on a plane to be inspected by a lattice-like image pickup element.

A plane accuracy data of a plane 1 to be inspected obtained from an interferometer 3 is a planarity data of the plane 1 to be inspected with reference to the reference plane 2a of a Fizeau flat 2 and care must be taken not to mix the undulation error of the reference plane 2a. If the reference plane 2a is an ideal plane, a plane 1a to be inspected represents true undulation. Assuming the intersection of diagonals of four sample points A, B, C and D forming one cell is M, the inclination angle at the intersection can be calculated from the length of the diagonal as a vector being defined by a γ and θ. More specifically, it is calculated as a vector γπ/4+θ from the magnitude of inclination angle απ/4 in the AD direction and inclination angle β3π/4 in the BD direction.