PURPOSE: To prevent bearing clearance from varying caused by the increase in temperature due to spindle rotation by making the main spindle body of an ultra precision grinding machine of silicon nitride ceramics and by making bearing metals made of NI-resist cast iron of which linear expansion coefficient is approximately same as that of the main spindle body.

CONSTITUTION: A main spindle body 41 made of silicon nitride ceramics forms a main spindle 40 with iron coupling members 42 and 43 fastened by a bolt 40. Bearing metals 20, 21, and 30 are made of NI-resist cast iron of which nickel content has been adjusted and of which linear expansion coefficient is approximately same as that of the body 41. A plurality of pockets 22 and discharge channels 23 are formed on a radial support faces 20a and 21a to which the body 41 is fitted keeping a slight clearance. Lubricating oil is supplied to the pockets 22 through channels 24, orifices 25, and channels 26. A flange section 41a is supported between thrust support faces 20b and 30a to form a plurality of pockets 31 and discharge channels 32 supplying the lubricating oil to the pocket 31 through channels 34, 33, and 34 and orifice 35.