To improve the thickness accuracy even when production parameters are changed by forming plural air feed paths on a disc-like plate and controlling the individual temperatures of auxiliary cooling air streams ejected from the air supply paths for performing the auxiliary cooling of a molten resin to be extruded from a die lip.

An auxiliary air ring 1 is of a disc-like shape with a cavity formed in the center part as viewed from above and a die lip is positioned in the cavity part. Inside the auxiliary air ring 1, a cluster of air feed paths 11 for performing the auxiliary cooling of a molten resin to be extruded in a tubular form are radially provided from the center part of an air ring 1 holder. Further, a heater 12 as a means for controlling the temperature of an auxiliary cooling air is installed halfway through the air feed paths 11 respectively. In addition, the turning ON/OFF of the heater 12 for heat generation and the setting of a heat temperature at a low or a high level are performed by a temperature sensor and a controller which supplies an electric power to the heater.