PURPOSE: To form the shape of a chamber engaging part with good accuracy, by forming the chamber engaging part brought into engagement with the middle of a timepiece case to the rising part of a core by semi-punching and embossing press processing using a die and a punch.

CONSTITUTION: A rear cover 5 is placed on a stand 16 and the projection 7a formed to the lower part of a die 7 is contacted with the outside of the rising part 6 of a core and press processing is performed from the direction shown by the arrow by a punch 8 having a downwardly inclined surface 8a. The rising part 6 is semi-punched so that the lower side surface 6c of the corner part 6b at the edge of the guide inclined surface 6a of the rising part 6 is embossed until contacted with the upwardly inclined surface 7b of the projection 7a of the die 7. By this method, the corner part 6b is projected to form a chamber engaging part 6'.