A method and an apparatus for measuring the thicknesses of film layers of different synthetic resins forming a composite multilayer film or sheet by utilizing infrared rays. Infrared rays of sample wavelengths, each equal to the infrared absorption wavelength of each film layer, and at least one reference wavelength different from the infrared absorption wavelength of each film layer are projected onto the multilayer film to be measured. Ratios are obtained between the amounts of infrared lights of the sample and reference wavelengths transmitted through the multilayer film. An operation is achieved for solving a simultaneous equation including the ratios and the infrared absorption coefficients of the film layers at the sample and reference wavelengths as coefficients and the thicknesses of the film layers as unknowns, thereby obtaining the thicknesses of the film layers.