An annular excess portion is disposed, a spline forming male mold is passed through a boss of a sheet metal-made rotary member in which a root portion of the boss and a peripheral portion are previously displaced, and the boss is set to a stationary mold with being fitted into a boss fitting hole of a rotary member holding mold. Thereafter, the annular excess portion is clamped between a pressing mold and the stationary mold to be deformed into a flat shape, and the root portion of the boss is squeezed into spline forming grooves to be plastically deformed, thereby forming a boss spline. According to the configuration, increase in thickness os a sheet metal blank is avoided, the strength of the boss is ensured, and weight reduction is achieved. The production efficiency can be remarkably improved, and cutting chips are eliminated, so that adverse influence on the working environment can be avoided.