Abstract: A junction bolt for joining members by use of clamping nuts engaged with the junction bolt is provided with an axial inner hollow hole in which a piston member is located so as to define a space between the outer peripheral surface of the piston member and the inner wall of the junction bolt. The space is divided by a sealing member into a pressurizing space into which a pressurized fluid is introduced and a temperature adjusting space into which a heat medium is introduced. In the bolt clamping force adjusting method, the temperature of the junction bolt is first adjusted to a predetermined temperature and an extending amount of the junction bolt to be caused when an aimed clamping force is applied at this predetermined temperature.