To offer a plane correcting method for a surface plate of a processing machine to admit a work made of a material having an extra-high hardness and requiring excellent flatness, parallelism, and surface roughness such as a ceramic block used as a pressure plate of polishing machine.

A lapping machine includes a rotatable surface plate 4 and a pressure plate 3 abutting to the surface plate, wherein a plate whereto a plurality of pellet-form diamond wheels 2 are affixed is attached to the surface of the pressure plate, and at least one of the surface plate and pressure plate is rotated while an oily processing liquid is being supplied, and thereby the surface of the surface plate is corrected. The pellet-form diamond wheel used should preferably be of metal bond type including diamond abrasive grains ranging from No.40 to No.400 as specified by JISR6001. It is also preferable that the pellet-form diamond wheels are arranged on plate in disc shape in its ring-shaped portion excluding the central part.