PURPOSE: To provide a solder joining method which does not induce flaws on the surface of an optical element in the process for production of the metal joined isolator.

CONSTITUTION: Metal joining is possible without requiring insertion of solder between the optical element 16 and a holder at the time of metal joining of the optical element 16 and the holder by forming an AuSn film 14 on the surface of the optical element by so-called physical vapor deposition methods, such as vapor deposition method, sputtering method and ion plating method, of an AnSn alloy.