for filtering device 63 for main extruder 60 a plurality of the filters are planted replaceably inside an external cylinder 86 as shown in fig1 on the other hand in the case where the filter element is used for filtering device 64 for auxiliary extruder 61 or 62 one such filter is attached replaceably in an external cylinder 87 as shown in fig1 it should be noted that filtering members 1 and 41 shown in fig1 and 4 can be composed by other than a sintered metallic screen laminated metallic screens calcinated laminated metallic screens sintered metallic felt fabric or sintered metal powder etc in the above mentioned figures p denotes a flow of molten resin star shaped cylindrical filter 65 which has an extremely large flitrating area as compared to the outside dimension thereof can be mounted inside the conduits which also serve to communicate the cylinders of auxiliary extruders 61 and 62 with the t die so as to supply molten resins accordingly no particular space for filtering device 64 is required as to filtering device 63 a number in association with the amount of resin supply of the star shaped cylindrical filter elements each having such an extremely large flitrating area are bundled in parallel to form filtering device 63 the thus constructed filtering device 63 is disposed downstream of the cylinder of main extruder 60 on the other hand the same star shaped cylindrical filter element is used as the elements for resin layer dedicated filtering devices 63 and 64 for forming different resin layers in a multi layer resin structure with this configuration all the resins for different layers.