PURPOSE: To increase the thickness of the circumferential edge of a fitting hole and prevent the increase in the weight by executing a plurality of specific works to a bottom plate part of a pulley made of sheet metal having a bottom plate part with a fitting hole for a boss and a belt winding part.

CONSTITUTION: A boring process to bore a hole part whose diameter is smaller than that of a fitting hole in a bottom plate part 6 of a plate W, a burring process to form a projecting part 8b of truncated conical shape where the diameter at the smallest diameter part is larger than the diameter of a hole part, and a thickness-increasing process to increase the thickness of the part where the projecting part is provided by flattening the projecting part 8b while the diameter of the smallest diameter part of the projecting part 8b formed in the burring process being kept larger than the diameter of the hole part are provided. This arrangement allows the thickness of the circumferential edge part of the fitting hole to be increased, leading to the reinforcement thereof.