PURPOSE: To reduce a bolt interval pitch by forming a fixing device of a bolt and a hydraulic nut having first and second elements, making axially movable relative to the first element by a fluid pressure in an annular chamber formed with the second element, relatively reducing the thickness of a chamber wall, and allowing it to be deformed in inner radial direction by fluid pressure.

CONSTITUTION: This fixing device 40 comprises a screw stamping part 57, a bolt 53 having a radial load supporting part 58, and a hydraulic nut 60 having a screw stamping part 63 operating in association with the screw stamping part 57 and elements 61 and 62 with a screwless part 65 used as their inner peripheral surface. An annular chamber 71 to introduce a hydraulic pressure from a fluid duct 72 is formed between the elements 61 and 62, and the element 62 together with the element 61 are moved axially in the chamber 71 to provide a tension to the bolt. The chamber 71 surrounds an annular surface 65, the radial inner wall of the chamber 71 is thin and deformed by pressurization, and the chamber 71 is supported on the load supporting part 58 of the bolt. Thus, a bolt interval pitch can be reduced by this thin wall structure.