PURPOSE: To provide a pelletized blowing agent compsn. which does not scatter, has good workability and is convenient for metering, by extruding a blend composed of a blowing agent having a specified decomposition temperature and a low-melting resin at a temp. not higher than the above decomposition temp.

CONSTITUTION: 10W100pts.wt. blowing agent having a decomposition temp. of 90W180°C such as benzenesulfonyl hydrazide is blended with 100pts.wt. low-melting resin such as a highly flowable low-density PE. The mixture is stirred at a high speed to obtain a blend which is then extruded at a temp. not higher than the decomposition temp. of the above blowing agent and pelletized to obtain a pelletized blowing agent compsn. Then 0.5W10.0pts.wt. said blowing agent compsn. is added to 100pts.wt. expansion molding resin such as PE or PP. The mixture is subjected to expansion molding by a conventional method, to obtain an expanded product.

EFFECT: Local discoloration due to excessive decomposition of the blowing agent can be prevented. A product having uniform color tone and fine appearance can be obtained.