A film thickness measuring device includes a spectroscopic sensor and a data processor, wherein the spectroscopic sensor measures spectroscopic data of a film coated on a substrate and the data processor obtains measured color characteristic variables from the measured spectroscopic data, compares the measured color characteristic variables with plural sets of theoretical color characteristic variables corresponding to plural sets of values, each set including one of plural values of thickness and one of plural values of index of refraction of the film, determines index of refraction of the film using the set of values corresponding to the set of theoretical color characteristic variables which minimizes a difference between the set of theoretical color characteristic variables and the measured color characteristic variables, and determines thickness of the film using the index of refraction of the film.