The invention provides a foamed body of thermoplastic resin having a high expansion ratio, desired shape and cells of desired diameter, especially fine cells. The foamed body of thermoplastic resin is produced by filling the thermoplastic resin containing a foaming agent into a cavity 31 of a mold 2a and thereafter enlarging the cavity 31 to expand the resin. In this process, the cavity 31 is completely filled with the resin as melted and containing the foaming agent, then held in its shape as filled with the resin and enlarged to an increased final cavity width upon the average temperature filled in the cavity 31 reaching the range from the melting point of the resin to the melting point plus 30.degree. C.