Disclosed is a method for preventing multiplication of algae which is characterized by coating the surfaces of solid materials with a silicone emulsion composition prepared by emulsion polymerization of:(1) 0.1 to 60% by weight of a reaction product prepared by reacting(A) 50 to 99.9% by weight of a polydiorganosilozane having more than one silanol group on the average in each molecule and having a viscosity of 5 to 10,000 cSt at 25.degree. C. with(B) 0.1 to 50% by weight of a product prepared by reacting (a) 1 mol of a compound represented by the formula: ##STR1## wherein Q.sup.1 is a monovalent group selected from the group consisting of a hydrogen atom, --CH.sub.3, --CH.sub.2 CH.sub.2 NH.sub.2 and --CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 NH.sub.2, R.sup.1 is a divalent hydrocarbon group having 1 to 4 carbon atoms, R.sup.2 and R.sup.3 are monovalent hydrocarbon groups each having 1 to 4 carbon atoms, and a is an integer of 0 or 1,with (b) 0.5 to 3.0 mol of a compound represented by the formula: ##STR2## wherein Q.sup.2 is an epoxy group-containing group selected from a glycidoxy group and an epoxycyclohexyl group, R.sup.4 is a divalent hydrocarbon group having 2 to 4 carbon atoms, R.sup.5 and R.sup.6 are monovalent hydrocarbon groups each having 1 to 4 carbon atoms, and b is an integer of 0 or 1, and (2) 1 to 50% by weight of a compound represented by the formula: ##STR3## wherein R.sup.7 and R.sup.8 are same or different substituted or unsubstituted hydrocarbon groups each having 1 to 50 carbon atoms, and n is an integer of 3 or more, in the presence of(3) 0.1 to 20% by weight of a quaternary ammonium salt surface active agent and(4) 20 to 90% by weight of water.