Biofilm formation inhibitor and biofilm formation inhibiting composition

Provided is a biofilm formation inhibitor that does not cause damage to piping or devices and that is safe on human bodies. The biofilm formation inhibitor contains, as an active ingredient, at least one quinone compound selected from the group consisting of hydroquinone substituted with a hydroxyl group or an alkyl group, and p-benzoquinone substituted with a hydroxyl group or an alkyl group, and C1-C5 alkylresorcinols. Further provided are: a biofilm formation inhibiting composition that contains the quinone-based compound and a chelating agent; a biofilm formation inhibiting method that comprises adding the quinone-based compound and the chelating agent in an aqueous liquid; a biofilm formation inhibiting composition that contains the quinone-based compound and an organic solvent; and a biofilm formation inhibiting method that comprises adding the biofilm formation inhibiting composition in an aqueous liquid.