In a process for producing a thermoplastic resin foam which comprises continuously heat-foaming a foamable thermoplastic resin sheet containing a heat-decomposable organic blowing agent with a heated gas, the improvement wherein a gas containing sublimation products generated by the heat-foaming of the foamable sheet is burned to decompose the sublimation products and obtain a treated gas having a substantially reduced content of the sublimation products, and the treated gas is then used as the heated gas for the heat-foaming of the foamable sheet; and a apparatus for producing a thermoplastic resin foam by continuously heat-foaming a foamable thermoplastic resin sheet, said apparatus comprising a heat-foaming chamber having an inlet portion for feeding the foamable thermoplastic resin sheet, an outlet portion for discharging the foamed sheet, a discharge port for a gas containing sublimation products generated by the heat-foaming of said foamable sheet, and a supply port for a heated gas, said discharge port and said supply port being connected by pipe lines to form a circulating pipe line, and said circulating pipe passage having a burning device for burning said gas containing sublimation products.