Description is made of an open cell polystyrene resin foam having a content of organic volatile matters of less than 100 ppm. The open cell foam is suitable as a core material for a sound insulating material or a vacuum heat insulating material.The open cell foam can be produced by melt-kneading a polystyrene resin having a content of organic volatile matters of 100 ppm or higher together with water in an extruder, sucking out the organic volatile matters as an azeotrope with water through a vent, injecting a blowing agent and kneading the injected mixture, and extruding the mixture into a lower pressure area to cause the resin to foam.