PURPOSE: To ensure proper hardness in the inner wall surface of a pin hole ranging from a crank pin inserting end to an arbitrary depth while ensuring slip torque in the inner wall surface from the depth to an opposite end to the crank pin inserting end by providing a carburized layer in the inner wall surface from said end to said depth.

CONSTITUTION: A crankshaft 51 connected to a crank arm 52 and a crank pin 55 for pivotably supporting a connecting rod 54 are provided. A pin hole 551 is provided in a position eccentric from the crankshaft 51 in the crank arm 52 and an end of the crank pin 55 is press fitted in the pin hole 551. Then, a carburized layer 6 is provided in the inner wall surface of the pin hole 551 from a crank pin inserting end to an arbitrary depth. Thus, the hardness of the peripheral wall surface of the pin hole 551 can be improved while slip torque between the inner wall surface of the pin hole 551 in the crank arm 52 and the crank pin 55 is maintained, so that the crank pin 55 can be firmly connected to the crank arm 52 and the durability of the pin hole 551 can be improved.