PURPOSE: To machine main bearing portions accurately and simultaneously by mounting, on crank pins, oscillation protecting disks whose centers coincide with the center of main bearing portions of a crankshaft, and which can freely be mounted on or dismounted from pins, and are supported freely in rotation by receiving rollers.

CONSTITUTION: Oscillation protecting disks 8 whose centers coincide with the center of main bearing portions 2 of a crankshaft (the center of a crankshaft) are mounted removably. Each of the disks 8 consists of unified one of two separated pieces, and their peripheries are supported by rotating rollers (not figured). By making rotation of the crankshaft 1, the main bearing portions 2 are machined with machining jigs. In this case, a plurality of the bearing portions 2 can be machined simultaneously.