PROBLEM TO BE SOLVED: To easily mount and demount a tool body to a tool, holder, and reduce the run-out of the tool body to the tool holder, by supplying the high oil pressure to a hydraulic supply passage formed on a fitting part, from an external. SOLUTION: A mounting part 14 comprises a step part 15 on a boundary part of a small diameter part 14a and a large diameter part 14b. An abrasive wheel 21 of small diameter, and a recessed fitting part 22 to be thermally inserted to the mounting part 14 of a tool holder 10 are integrally molded in a tool body 20. The fitting part 22 comprises the first fitting hole 22a and the second fitting hole 22b, and a step part 23 is formed on a boundary part thereof. When the fitting part 22 is fitted to the mounting part 14, a prescribed space is formed between the step part 23 and the step part 15. When the high oil pressure is supplied to the space A, an oil film of high pressure is formed on a fitting face of the mounting part 14 and the fitting part 22, and acts as a lubricant, whereby the slippage and the inclination in an axial direction, of the fitting part 22, are corrected. Accordingly, the tool unit 1 free from the run-off can be obtained. Further, the tool body 20 can be easily removed from the tool holder 10 by the oil pressure.