To coagulate the thus-prepared latex, it is preferable to use at least one amine compound selected from tertiary amines, quaternary ammonium compounds, and compounds represented by the following formula (2): ##STR3## wherein R3 is a hydrocarbon group, R4 and R5 independently represent divalent hydrocarbon radicals, r and s are integers of 1 to 60, and t is 0 or 1.