A ruthenium film forming method includes a deposition process of introducing a mixed gas of a ruthenium carbonyl gas and a CO gas into a processing vessel 1 by supplying the CO gas as a carrier gas from a CO gas container 43 configured to contain the CO gas into a film forming source container 41 configured to contain ruthenium carbonyl in a solid state as a film forming source material, and forming ruthenium film by decomposing the ruthenium carbonyl on a wafer W; and a CO gas introduction process of bringing the CO gas into contact with a surface of the wafer W by introducing the CO gas directly into the processing vessel 1 from the CO gas container 43 after stopping the introducing of the mixed gas into the processing vessel 1. The deposition process and the CO gas introduction process are repeated multiple times.