To improve positioning accuracy and processing accuracy of a main shaft head, that is, a tool by smoothly moving a main shaft head.

An attaching plate 83 having a substantially L-shaped section is attached along an outer peripheral surface of a stainless steel cover 80 fixed to an outer peripheral surface of a main shaft head 60. A resin sealing member 81 is opposed to the outer peripheral surface of the stainless steel cover 80 and positioned by a bolt 84 screwed with an attaching seat 83a of the attaching plate 83. The sealing member 81 is pressed against the outer peripheral surface of the stainless cover 80 by a coil spring 82 arranged between the attaching seat 83a and the sealing member 81.