Provided are a thin film for optical element as a single-layer thin film which contains a Si simple substance, a Si compound excluding Si alloy, and a metal or metal compound, a method of manufacturing the thin film for optical element, and an optical element including the thin film for optical element. Further provided are an inorganic polarizing plate including a reflection suppressing layer composed of the thin film for optical element, a method of manufacturing the inorganic polarizing plate, and an optical device including the inorganic polarizing plate.