Biofouling and biofilm formation on inside walls of pipes where various liquids flow have caused varoius netagive industrial problems so far, since it would lead to corrosion, slimes and scales to deteriorate the functions such as heat exhcnage capability and materials qualities/characteristics themselves. As the countermeasure, various biocides and chemical cleaners have been used to control liquid qualities and to prevent those negative industrial problems. Various acids, alkaline agents, surfactants etc. have been used conventionally so far. At this point, hypoochlorous acid and some organic agents seem to be excellent chemical cleaners. However, the former might induce corrosoin on the substrate walls. On the other hand, the latter might evaoprate during the use, evne though any of them might show higher performances. In this experiment, we focused on some ionic liquids. They might be able to overdome those defencts mentioned above. When we come to think about the applicability of ionic liquids to practical industrial purposes, the price and the environmental friendliness may be serious problems in the futuure. In those cases, it will be key points how we could minimize the amount of use for the purpose. Therefore, we tried in this experiment to confirm how very small amounts of some ionic liquids could control biofilm formation. To produce biofilms articially with good reproducibility, we used a loop-type laboratory biofilm reactor which we designed and produced in our laboratory. Two kinds of ionic liquids were used for the experiments. 1-Butyl-3-methylimidazolium Tetrafluoroborate(BMImBF₄) and Tributhylocthylphosphonium chloride(P4448Cl). Small amount of them were added into LBR filled with tap water. The liquids including ionic liquids were circulated by a pump and biofilms formed on specimens placed with a stick-type jig, being parallel to the flow direction. After a couple of days passed, the specimens were talken out from the system and the material surfaces were observed. Some types of liquids without ionic liquids were used as control and all of these specimens after exposures in the LBR were analyzed by optical micrscopes, staining by crystal violets and Raman spectroscopy and discussed. The effect of ionic liquds were obviously recognized during the initial stage of biofilm formations. However, ionic liquids also had the difficulty to disolve the solidified biofilms contained lots of calcium carbonate produced at the second half of biofilm formation process.