The present invention provides is a thin film analyzing method which can be applied to various fields, and which makes it possible to detect and analyze in a simple manner, with high precision, a distribution of a specific component in a thin film formed on a support. The method for analyzing a constituent of a thin film comprises a cutting step of cutting the thin film obliquely, and an analyzing step of analyzing a specific component in the cut section of the thin film. In this cutting step, the thin film is preferably cut with a microtome to which a cutting edge made of glass is fitted knife made of glass. The analysis of the distribution of the specific component in the cut section is suitably analyzed by TOF-SIMS or .mu.-ESCA. The method is particularly useful for analyzing an image recording layer of a planographic printing plate precursor which comprises a water-insoluble and alkali-soluble resin, an infrared ray absorber, and a colorant.