A synthetic latex is prepared by charging a portion of water and a monomer as raw materials in a reactor in a ratio of monomer/water, at which a maximum ultimate pressure in said reactor that is attained if an initial composition in said reactor before the initiation of the polymerization is polymerized from a polymerization initiation temperature in an insulated state, is within a practical proof pressure of said reactor; initiating the polymerization; adding dropwise the remainder of said water and said monomer during the polymerization, continuously or intermittently, and performing the emulsion polymerization. As a result, the emulsion polymerization can be stably performed while controlling the temperature and pressure in said reactor in a runaway reaction in all processes of the method of preparing the synthetic latex, thus making it possible to improve the productivity.