body by the blow molding as described above for example the foamed molded body may be obtained by vacuum forming that involves attaching the extruded foamed resin to the mold 12 by suction to mold the foamed resin into the predetermined shape fig4 is an enlarged diagrammatic view illustrating a portion near a surface of the mold illustrated in fig3 the protrusion to be formed on the surface of the duct is formed in such a manner that a shape of a groove 12 a on the mold 12 illustrated in fig4 is transferred onto the surface of the parison 11 the groove 12 a of the mold 12 is formed such that a width w and a depth d satisfy the following relations herein the width w of the groove is measured as a clearance between ends f 1 and f 2 of mold plane portions f located on both sides of the groove in a vertical section of the groove i e a cross section which is perpendicular to a direction of extension of the groove moreover the depth d is measured as a distance from a deepest position in the groove to a straight line connecting between the ends f 1 and f 2 of the mold plane portions f located on both the sides of the groove for example the groove 12 a of the mold 12 is formed to satisfy relations d 1 mm and w 3 mm as illustrated in fig2 in this embodiment both the sides in the groove width direction are formed to have an arc shape which is 1 mm in radius moreover the middle portion in the groove width.