PURPOSE: To remove a biofilm from an under water surface by allowing the biofilm composed of microorganisms to contact with a prescribed amount of a stabilized chlorine dioxide aq. soln., the nallowing the biofilm to contact with the prescribed amount of a nutrientsource.

CONSTITUTION: The biofilm composed of the microorganisms existing at the under water surface in an aq. medium is brought into contact with the stabilized chlorine dioxide aq. soln., then the biofilm is brought into contact with at least one kind nutrient source to remove the biofilm. At this case, an effective amount of the nutrient source for forming more than one kind acidic compd. in the biofilm is supplied. And the stabilized chlorine dioxide aq. soln. is supplied in the effective amount for sterilizing the microorganisms in the sufficient number of the biofilm for removing the biofilm from the under water surface when the film is activated with the acidic compd. formed with the microorganisms.