PURPOSE: To provide a method and device for measuring optical characteristics of a thin film to measure the optical characteristics of heterogeneous film or multi-layer film easily.

CONSTITUTION: A substrate 8 on which a thin film is to be formed, and a monitor plate 9 are fitted onto a rotary plate 4 placed in a vacuum tank, and a shielding plate 31 is provided thereto to prevent the film formation against the plate 9. Further, the plate 31 is turned synchronously together with the plate 4 and the phase of the plate 31 against the plate 4 is adjusted to expose the shielded part of the plate 9 successively. The optical characteristics of a single-layer film which is formed on a newly exposed part is measured so that the optical characteristics of multi-layer film formed on the substrate 8 may be calculated.