in a process for melting , extruding , and forming a thermoplastic synthetic resin , wherein a deckle mounted on a die device of an extruder is moved lengthwise of a slit in the die to thereby vary the length of the slit and to adjust the width of a flat film being extruded , the improvement comprising stopping the leaking of the molten resin around the deckle portion by forcibly cooling the deckle with a cooling medium , such as air , water or the like , so as to increase the viscosity of the molten resin around the deckle , whereby a decreased fluidity of the molten resin occurs .