To provide a spindle unit mechanism mounted with a rotary tool which forms a gap between an inner circumferential surface of a radial bearing and an outer circumferential surface of a rotary spindle into an adequate gap corresponding to the rotational speed of the rotary spindle.

The spindle unit mechanism is provided with the radial bearing 6 and a thrust bearing 8 which are arranged in a spindle housing 3 and fluidly support the rotary spindle 4 by forming a fluid layer between themselves and the outer circumferential surface of the rotary spindle and a high pressure fluid supply means 10 for supplying a high pressure fluid to the radial bearing and the thrust bearing of a spindle unit 2. The radial bearing is formed of an elastically deformable material and comprises a cylindrical bearing sleeve having at an inner circumferential surface a plurality of fluid jet holes communicating with the high pressure fluid supply means. The spindle unit mechanism has a gap adjustment means which reduces a diameter by elastically deforming the cylindrical bearing sleeve corresponding to the rotational speed of the rotary spindle and adjusts the gap between the inner circumferential surface of the cylindrical bearing sleeve and the outer circumferential surface of the rotary spindle.