PURPOSE: To work a piston ring with high accuracy in a short time by executing groove working by a press work, in case of performing the groove working to a piston.

CONSTITUTION: On one face of a columnar fixed base 1, a groove part 11 in which a piston ring 2 is contained is formed, and this groove part 11 has almost the same width as that of the piston ring 2, and its depth is formed to almost the same as thickness of the piston ring 2. On the bottom face of the groove part 11, a projecting part 12 of about 1/3 of depth of the groove is provided on plural parts. In case of forming a notch part in the piston ring 2, the ring 2 is placed in the groove part 11 of the fixed base 1. In this case, the ring 2 is supported by the projecting part 12 and projected a little from the upper face of the fixed base 1. Thereafter, when the fixed base 1 is pressed by using a press base 3, as for the ring 2, the part supported by the projecting part 12 becomes a recessed state.