An apparatus for forming a thin film by repeating, plural times, a cycle including supplying and adsorbing a precursor gas onto a substrate and generating a reaction product by allowing the precursor gas on the substrate to react with a reaction gas, which includes: a main precursor gas supply part for supplying the precursor gas; a reaction gas supply part for supplying the reaction gas; an adjustment-purpose precursor gas supply part for supplying an adjustment-purpose precursor gas to adjust an in-plane film thickness distribution of the thin film; and a controller for outputting a control signal to execute a step of forming the thin film using the main precursor gas supply part and the reaction gas supply part, and subsequently a step of supplying the adjustment-purpose precursor gas from the adjustment-purpose precursor gas supply part to compensate for a film thickness of a portion having a relatively thin film thickness.