Abstract: A spiral die assembly can obtain a homogeneous pipe body, an inflation film with/without drawing and a functional film by dispersing molten resin homogeneously, and the spiral die assembly is capable of being applied to various molten resins and extruding conditions. The spiral die assembly comprises a main body 1 of said spiral die assembly, a mandrel 2 having a spiral groove 3 around its outer surface inserted on the inside of the main body and a rotatable ring 5 having a desired width fitted along the outer surface of the spiral groove 3 and fitted in a portion of a die cylinder which forms the inner surface of the main body 1 in a range corresponding to both ends of the spiral groove 3.