PROBLEM TO BE SOLVED: To prevent a molten resin coating film from being affected by a necking phenomenon by providing specified sets of endless belt devices which clamp both end parts of the molten resin coating film to be extruded from a T-die between the T-die and a coater roll. SOLUTION: An endless belt 9 is hooked around two right/left sets of pulleys 5 which are installed right below a T-die 2 in such a manner that the pulleys 5 have rotary shafts in parallel with a slit formed on the lower face of the T-die 2 and clamp a molten resin coating film end part 3a and further around a coater roll 4 which is installed right below the pulleys 5 in such a manner that the coater roll 4 has a rotary shaft in parallel with a slit formed on the lower face of the T-die 2. A heated molten resin is extruded as molten resin coating films 3, 3a from the slit formed on the lower face of the T-die 2. Then a necking phenomenon occurs across the width of the coating films 3, 3a, which contract in width, dropping after being suspended. However, the two right/left sets of the endless belts 9 provided in parallel with the molten resin coating films 3, 3a right below the T-die 2 clamp the molten resin coating film 3a. Thus it is possible to prevent the width contraction from occurring beyond the existing level.