PURPOSE: To form a receiving seat without making the assembly complicated and without increasing the number of component parts, by pressing the inside bottom part of a case formed by spinning, and forming a receiving seat of a bottom valve in the inside.

CONSTITUTION: For instance, a damper case 1 of oil pressure buffer for automobile is provided so as to be like a long pipe by its spinning, and also in a state that a bottom part 2 is closed. Its inside bottom part is formed somewhat thick, and leaves thickness required for forming a receiving seat. Subsequently, a mandrel 3 being a die whose tip part is provided for forming the receiving seat is inserted into the case 1 through a press machine, etc., the inside bottom part of the case is pressed against a die 5, and a receiving seat part 4 is formed. In this way, a bottom valve can be installed directly, therefore, parts for the receiving seat and its inside fitting and fixing work become unnecessary.