To obtain an activator for emulsion polymerization, which gives good polymerization stability in producing an emulsion and comprises a lowly foaming nonionic activator by subjecting a specified alcohol to random or block addition reaction with ethylene oxide and propylene oxide and then subjecting the adduct to addition reaction with ethylene oxide.

An alcohol of the formula R-OH (R is an 8-20C linear or branched alkyl, or alkenyl) is subjected to random and/or block addition reaction with ethylene oxide and propylene oxide, and the adduct is then subjected to addition reaction with ethylene oxide to give an activator for emulsion polymerization, which comprises a nonionic activator of the formula RO-[(EO)x.(PO)y]-(EO)z-H EO is oxyethylene; PO is oxypropylene; x and z are each the average number of ethylene oxide molecules added and 1 or greater, provided that x+z≥8; y is the average number of propylene oxide molecules added and 1 or greater, provided that it is smaller than x+z; and [(EO)x.(PO)y] and (EO)z are combined together in block form}.