A mold equipped with a core having a plurality of branching columnar distal portions in the extrusion direction and a die having an aperture surrounding the distal portions. A softened flexible resin is injected from one end of a resin flow path formed between the core and the die and extruded in order to form a pipe material having a plurality of circular liquid flow paths which are mutually independent and have the same inner diameter. The distal portions of the core shaped as a row of circular distal portions aligned on the same axis, where the two edge distal portions have a smaller diameter than the distal portions disposed between the edge distal portions, while the portions of the aperture surrounding each of the distal portions have the same diameter.