To obtain an excellent-quality foam while diminishing the amount of the fluorocarbon used or dispensing with its use on an account of the environmental pollution by forming a closed-cell extruded foam of an alkenylaromatic synthetic resin by using CF3CH2F.

An extruded closed-cell foam made of an alkenylaromatic synthetic resin and containing 1,1,1,2-tetrafluoroethane is provided. The blowing agent is usually contained in an amount of about 3-30 pts.wt. per 100 pts.wt. alkenylaromatic synthetic resin. It is desirable in most cases that a nucleator is added to regulate the cell size. Talc, calcium silicate, indigo, etc., are nucleators desirable to regulate the cell size. The obtained foam generally has a cell size of about 0.05-3.5mm and a density of about 16-80kg/m3. Further, it has a minimum thickness of cross-section of 1/2 inch (1.27cm) and a minimum area of cross-section of 8 square inch (52cm2).