A metal nanodot formation method includes: loading a target substrate inside a processing container of a processing apparatus; depositing a plurality of metal nanodots on a surface of the target substrate by a sequence of: supplying a CO gas from a CO gas container which stores the CO gas into a raw material container which stores a metal carbonyl compound; generating gas of the metal carbonyl compound; introducing the generated gas of the metal carbonyl compound as a mixture gas containing the CO gas into the processing container; and decomposing the metal carbonyl compound on the target substrate, and directly introducing the CO gas from the CO gas container into the processing container, in a state where the introduction of the mixture gas into the processing container is stopped, such that the CO gas is brought into contact with the metal nanodots on the surface of the target substrate.