PURPOSE: To form a highly accurate inclined face by measuring the distances of the intersection lines between planes which are parallel and perpendicular to the reference face of a workpiece and an inclined plane, in optional places, and correcting differences between them when forming an inclined face of a prism, etc.

CONSTITUTION: The reference plane A of a workpiece 1 is stuck to a plane parallel plate machining jig 2, and its plane 4 is machined in parallel, with the bottom face 3 of the jig as the reference. Then, the whole is mounted on a first inclined face machining jig 5, and a first inclined face 7 is machined with the bottom face 6 of the jig 5 as the reference. In this case, the distances of an intersection line 9 between the parallel plane 4 and a perpendicular plane 8 and an intersection line 10 between said plane 4 and the first inclined face 7, are measured at optional positions 11, 12 and machining is carried out up to prescribed dimensions while correcting errors to obtain the same distances, to complete the first inclined face. And, a second inclined face 15 is also formed in a similar manner to above, from the intersection line 16 between the second inclined face 15 and the first inclined face 7 and said intersection line 9 or 10. This finishing order should not necessarily be the same order and, thus, a highly accurate inclined face can be formed.