A thin film producing method in which the wafer film forming processing for a wafer to be a product may be carried out efficiently to shorten the processing time and to raise the operating ratio of the device. In a thin film deposition method using a single wafer processing for forming a thin film by chemical reaction under heat, a pseudo-process is provided which operates to suppress variations in the film thickness caused by the temperature in a reaction chamber 11. This pseudo process is the pre-heating processing of heating the reaction chamber 11 before actually charging the wafer W into the reaction chamber 11.