PURPOSE: To make it possible to improve the flame retardancy of the obtained foamed resin without detriment to its heat resistance and mechanical properties, by adding a solution of a specified flame retardant in an organic solvent and a blowing agent to a styrene resin and expanding the mixture by heating.

CONSTITUTION: 100pts.wt. styrene resin is mixed with a solution of 0.1W10pts.wt. compound (A) of formula I or 0.1W10pts.wt. in total of a compound of formula I and a compound of formula II in an organic solvent and a blowing agent, and the mixture is expanded by heating. In formulas I and II, X is chlorine or bromine and the X groups are not necessarily the same, Y is a 1W6C alkylidene, a 6W12C cycloalkylidene, -O-, -CO-, -S-, -SO2- or a direct bond, R1 and R2, which are independent of each other, are each H or methyl, and m and n, which are independent of each other, are integers of 1W4.