To obtain a resin foam in which the constituent triblock polymer is not crosslinked so that the foam may be amenable to repair when broken and to recyclement after being disused and which still has improved vibration- damping performances by mixing a triblock copolymer of styrene with vinylisoprene with a styrene resin and foaming the mixture.

A mixture obtained by melt-mixing 30-80 wt.% styrene/ vinylisoprene triblock copolymer with 70-20 wt.% styrene resin in the screw cylinder of an extruder is foamed by extrusion into air through a die attached to the front end of the cylinder while a volatile blowing agent having a boiling point lower than the melting points of the resins or a gaseous blowing agent is injected into the mixture to obtain a synthetic resin foam having a density of 0.6-0.035 g/cm3. When this foam is the one having a high expansion ratio and therefore having a large volume, it can be utilized in general-purpose applications.