Corrosion are one of the paramount properties industrial materials surfaces should have. In recent years, many researchers often point out corrosion particularly in atmospheric environments initiates with the thin water film on materials surfaces. However, few researchers recognize the water component could be kept by biofilm formation through bacterial activities. Biofilm forms by aggregation of bacteria on material surfaces and often causes corrosion by the formation of concentration cells on materials surfaces. From the viewpoint, it should be moderated properly from the viewpoint of corrosion resistance. In this paper, we devised a special reaction container where biofilm forms from ambient germs in a water circulation system. Then some metal specimens were set in a column, a part of the water circulation system. The alternative electro-magnetic field was applied to the system and the change of biofilm formation was investigated under various experimental conditions.