A process for an extruded resin foam, comprising extruding a foamable composition comprising a melted polystyrene resin and a physical blowing agent through a die to obtain a polystyrene resin foam having a thickness of at least 10 mm and a transverse cross-sectional area of at least 50 cm.sup.2, wherein said extrusion is performed so that the ratio Mz/Mn of a Z average molecular weight Mz of the polystyrene resin foam to a number average molecular weight Mn of the polystyrene resin foam, each measured by gel permeation chromatography, is 8.0 or more.