PURPOSE: To obtain the subject extrusion-foamed material imparted with excellent printability without lowering the heat-resistance and suitable as a food container for microwave oven by extrusion-foaming a copolymer composed of a specific ratio of methacrylamide, styrene and (meth)acrylic acid in the presence of a foaming agent.

CONSTITUTION: The objective extrusion-foamed material free from shape-change even at a high temperature (≥80°C) and having low thermal dimensional change is produced by extrusion-foaming (A) 100 pts.wt. of a methacrylamide copolymer composed of (i) 5-60mol% of methacrylamide, (ii) 39-94mol% of styrene and (iii) 0.1-20mol% of (meth)acrylic acid in the presence of (B) 2-30 pts.wt. of a foaming agent.