PURPOSE: A latex of an unsaturated acid-a specific alkyl acrylate copolymer and an inorganic electrolyte are added to a synthetic rubber latex to grow the particles of the synthetic latex stably and uniformly.

CONSTITUTION: To (A) 100pts.wt. of a synthetic rubber latex such as polybutadiene latex, which is adjusted in pH to higher than 7, are added (B) 0.01W10pts., calculated as the solid part, of a compolymer latex that is obtained by emulsion polymerization of a monomer mixture consisting of (a) 3W30wt% of at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid and crotonic acid, (b) 97W50wt% of 1W12C alkyl acrylate and (c) 0W47wt% of other monomers, e.g., n-butyl acrylate-methacrylic acid copolymer latex and (C) 0.05W4pts. of an inorganic electrolyte such as sodium chloride.