PURPOSE: To eliminate an sanitary problem and to enable the economical production of the subject product, by a method wherein a given amount of water content is existed in a given unsaturated nitrile resin powdery object, said powdery substance is held in a sealing condition at the specified temperature and period of time, and is extruded or extrusion-molded.

CONSTITUTION: There exists 0.7W2.5wt% of water content in an unsaturated nitrile resin powder whose apparent specific gravity is 0.2W0.45 and which contains 40W90wt% of an unsaturated nitrile component. After the resin powder is held in a sealing condition at 0W80°C for 10hr or more, it is extruded or extrudion-molded to produce the foamed product. A method in which a given amount of water content exists in the unsaturated nitrile resin powder, causes the resin powder to dry so that the mixture is performed by means of a blender so that a watercontent becomes 0.7W2.5wt%, can be adopted. One of methods to hold in a sealing condition is a method in which a polyethylene inner paper bag, a metallic container, etc. are filled with the resin powder.