A vapor deposition process for depositing TiO2 and a vapor desposition process for depositing SiO2 are alternately repeated in a multi-layer film forming process. A refractive index that a thin film formed by each vapor depositing will provide is individually determined prior to each relative vapor depositing, and vapor deposition control data is prepared based on such a refractive index. Each vapor deposition is controlled by using a relative vapor deposition control data thus prepared. Therefore, each vapor deposition process can be accurately controlled according to the refractive index of a thin film even if repeated vapor deposition processes change the refractive index. Accordingly, a multilayer film having desired optical characteristics can be formed.