An interferometer previously developed for measuring instantaneous film thickness of the order of 10 &mgr;m–1 mm [T. Ohyama, K. Endoh, A. Mikami, and Y. H. Mori, Rev. Sci. Instrum.59, 2018 (1988)] has been improved with respect to both facility of handling and accuracy of thickness determination. The improved version of the interferometer uses an expanded laser beam focused aslant on the subject film. The rays reflected from the front and rear surfaces of the film form interference fringes on a screen, which are then recorded in the form of a single snapshot interferogram or as a set of serial interferograms to be geometrically analyzed. Experiments with thin glass plates as sample films verified the expected advantages of the new version over the original one. The improved version of the interferometer was also tested on a liquid film asymptotically thinning as it drained down a vertical plate. The test indicated that the accuracy and precision of the new improved version far exceed those of other transient film‐thickness measuring techniques. ©1996 American Institute of Physics.