PURPOSE: To partially form the floating single crystal thin film in a floating thin film, the floating single crystal thin film comprising a part of a single crystal substrate irrespective of the concentrations and conductivities of impurities.

CONSTITUTION: The method for forming the floating single crystal thin film comprises the first step for employing a surface having the lowest etching rate against an anisotropic etchant (A) as the surface of a single crystal substrate (1) and for surrounding the outer periphery of a floating single crystal thin film (10) with an outer peripheral material (I) capable of being slightly etched with the anisotropic etchant (A), the second step for covering a part or all of the surface of the single crystal substrate (1) with a covering material (J) capable of being slightly etched with the anisotropic etchant (A) and an isotropic etchant (B), the third step for etching the outer periphery of the region surrounded with the outer peripheral material (I) or a part of the lower single crystal substrate (1) with the isotropic etchant (B) to form an etching groove (3), and the fourth step for etching a part of the single crystal substrate (1) with the anisotropic etchant (A) by utilizing the etched groove (3) to form a floating single crystal thin film.