A mold-removal hole (60) formed upon forming a locking portion (39) and communicating with a tab insertion hole (31) is formed before a locking portion (39) by partly cutting a wall of the tab insertion hole (31). As this mold-removal hole (60) is formed, preventing portions (70) for preventing a displacement of the tab (22) inserted into the tab insertion hole (31) toward the mold-removal hole (60) are formed on the wall of the tab insertion hole (31) at the lateral sides of the mold-removal hole (60) and at the opposite widthwise outer sides of the locking portion (39). By displacing the position of the locking portion (39) toward one widthwise side from the center of the cavity (32), the preventing portion (70) provided at the other side is set to have an increased area of engagement with the tab (22).