PURPOSE: To enable high-accuracy finishing without any influence of thermal expansion at the time of polishing by joining as one body together a heat resisting glass level block, the surface of which is flat and a sub-plate made of low thermal expansion material one and rotating the joint body through a rotary shaft.

CONSTITUTION: A sub-plate which has a flat surface and is formed by low thermal expansion material is joined to a heat resisting glass 1 has a flat surface as one body to be rotated on a rotary shaft 3. The sub-plate 2 is manufactured by low thermal expansion material having a linear expansion coefficient of 1.0-5.0×10-6, for example, so that at the time of polishing, the influence of thermal expansion is eliminated to prevent unevenness in working due to the deformation of a heat resisting glass level block having a flat surface. Thus, in the case of polishing a work such as a semiconductor wafer, a glass substrate for a liquid crystal cell or the like by using fine abrasive grains, scratch casued by the shape and material quality can be prevented so as to accomplish high accuracy working.