Provided is a surface treatment method for performing machining such as cutting work, grinding, and electrical discharging to a plate-like material with two- or three-dimensional deformation to realize a uniform thickness. This method includes the steps of mounting the plate-like material on a surface plate, setting a coordinate axis in a plane direction of the plate-like material to be X, Y and setting a coordinate axis in a height direction of the plate-like material to be Z, virtualizing a surface containing an origin of the measured Z direction, measuring a height Z1-n from the origin in an arbitrary plane position, and inclining and cutting the plate-like material so that an absolute value of a difference between a maximum value Zmax and a minimum value Zmin of the obtained height data will be minimum. Although a ceramic sintered plate such as a sputtering target or a metal plate prepared by metal rolling or forging, in most instances, is subject to two- or three-dimensional deformation as a result of thermal stress or machining stress during the manufacturing process, this invention is able to obtain a flat plate-like material having a uniform thickness and minimal machining costs from a plate-like material with two- or three-dimensional deformation.