To generate residual compression stress in a boundary area between a seat face of a cap and a side wall of a crankshaft neighboring the seat face while suppressing increase in manufacturing cost.

A cap-side circular arc face 31 formed to a cast member 3A as a cap and holding a pin part of the crankshaft is pressed in a Y direction opposed to a direction to which the cap-side circular arc face 31 relating to an extending direction of a bolt hole 32 is opened, and engaging parts 34 formed to each of side walls positioned on both sides of the cap-side circular arc face 31 in the cast member 3A is pressed in an X direction opposed to the Y direction, so that tensile stress is generated in an area B where the seat face 33 in contact with a nut is formed. With a state where the tensile stress is generated in the area B where the seat face 33 is formed, the seat face 33 is processed and formed.

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