A CaF.sub.2 buffer layer (3) is formed on a CaF.sub.2 (111) substrate (2) by an MBE method. Furthermore, a CuCl thin film is grown on the CaF.sub.2 buffer layer (3) by the MBE method while irradiating it with an electron beam to form an electro beam irradiation film (1a). Subsequently, a CuCl thin film is grown by the MBE method without the irradiation of electron beam to form an electron beam non-irradiation film (1b), thereby thus forming a CuCl thin film (a) including the electron beam irradiation film (1a) and the electron beam non-irradiation film (1b). Consequently, a CuCl thin film (1) exhibiting high planarity and crystallinity can be formed.