PURPOSE: To ease stress and raise fretting corrosion fatigue limit by providing a curved face of a specified curvature to the opening of a fitting hole in the fitting hole in a combination member comprising a body to lock shafts by means of the fitting hole.

CONSTITUTION: A shaft 2 is fitted in and locked to the fitting hole of a body 1, and a curved face 5 is formed in the opening 4 of the fitting hole 3 at a specified curvature. When shearing force Q is applied to the shaft 2, the shaft 2 is sag down, and the point abutting on the curved face 5 continuously moves from a point (a) to point (c) through a point (b), and the length at a moment continuously become short as L1→L2→L3. Therefore, shearing force Q continuously increases due to the shortening of shearing span L with the deformation of the shaft 2. Since the shaft 2 deforms while attracting the energy of with the lapse of time, it becomes strong to impact loads and the rigidity of the shaft 2 increases. Since the fulcrum continuously moves, the stress of the shaft 2 can be prevented from concentrating and fatigue failure can be avoided. A fitting hole can be formed by providing a specific curvature to the outer envelope of the cutting edge of a boring tool.