PURPOSE: To prevent scoring from occurring between a rotary shaft and a bearing part by a method wherein the rotary shaft on the side of a front-side block or one of bearing holes provided in the block supporting the rotary shaft is provided with a tapered surface such as to increase a clearance in an outward direction.

CONSTITUTION: The bearing hole 5A rotatably supporting the rotary shaft 7A of a rotor on the side of a front-side block is provided with the tapered part 5B such as to gradually increase the clearance in the outward direction toward a front head 3, and the right-side part of the bearing hole 5A is contracted when it is cooled by a coolant gas of low pressure and low temperature which is sucked in through a suction port 11. However, when the coolant gas is sucked into a cylinder 4 through suction ports 8, 8 and compressed by the rotation of the rotor 7, the temperature and the pressure of the gas are increased and the left-side part of the bearing hole 5A is expanded. Although the clearance between the rotary shaft 7A and the bearing hole 5A is slightly reduced as a result of the thermal deformation, the tapered part 5B of the hole 5A serves to leave a sufficient clearance for forming a film of a lubricating oil between the shaft 7A and the hole 5A. Accordingly, it is possible to prevent the scoring from occurring between the rotary shaft 7A and the bearing part.