PURPOSE: To heighten the efficiency of foaming by preventing CO2 formation occuring just after the end of mixing, by coating a blowing agent consisting of an inorganic CO2-producing agent and a weak acid with a specified coating agent, mixing the coated blowing agent with a polyolefin resin, and extruding the mixture.

CONSTITUTION: One or both of blowing agent components: (1) an inorganic CO2- producing agent, e.g., sodium bicarbonate and (ii) a weak acid, e.g., citric acid, are coated with a coating agent, m.p. 40W120°C, e.g., wax, and both components are added to a polyolefin resin. The resulting composition does not generate CO2 because components (i) and (ii) are separated from each other and do not come in contact with each other. When this composition is extruded, the coating agent melts at the extrusion temperature, components (i) and (ii) come in contact with each other and efficient expansion by CO2 takes place.

EFFECT: It is possible to obtain foamed sheet having a smooth surface and fine and dense cells.