A workpiece enlarging method is provided, by which an enlarged portion can stably be formed on a portion of a workpiece such as a shaft member, and a drive force required to form the enlarged portion can be reduced. The workpiece enlarging method forms the enlarged portion on the workpiece (W) by applying, with respect to the workpiece (W), alternating shear energy in a transverse direction thereof so as to suppress deformation of the workpiece (W) within an elastic limit while applying compression energy that produces compressive stress equal to or greater than an initial yield strength to increase internal energy of the workpiece (W), thereby decomposing a portion of the increased internal energy by the alternating shear energy and deforming and enlarging the enlargement intended area of the workpiece (W) with the assistance of the decomposed energy obtained by the decomposition.