PURPOSE: To obtain a die for variable cross section extrusion and variable cross section extruding method capable of producing a formed part, etc., of round cross section changing its diameter in the longitudinal direction.

CONSTITUTION: A pair of dies 10A, 10B are arranged respectively turnably around axial lines 11A, 11B parallel each other, circular arc faces 12 (outer circumferential face for cylindrical shaped die), which are brought into contact each other along while turning the dies 10A, 10B around each axial line 11A, 11B, are formed to each die 10A, 10B, groove parts 13, which are extended in the circumferential direction of each circular arc face 12 and in which the cross section is continuously changed, are formed to each circular arc face 12, a closed cross section shape extruding hole 14 is formed by the groove parts 13 of both circular arc faces 12. As the groove part 13 is made to a cross section semicircle groove continuously changing its radius along the circumferential direction of circular arc face 12, the extruding hole 14 is formed to round, by turning the dies 10A, 10B during forming, a round cross section formed part changing its diameter in the longitudinal direction is produced.