To provide a metal tube end diameter-reducing method and device therefor in which a diameter-reduced part of a predetermined reduced diameter can be formed straight without generating any buckling even when the diameter reduction ratio is large and R of a tapered part is small.

A die 1 is connected to a tapered part 12 of a large-diameter straight tube in which the inside diameter of an inlet part 11 is substantially as large as the outside diameter of a worked tube (a metal tube) 3, and the inner part thereof is gently diameter-reduced, and has a worked hole in which the inner side thereof is gently connected to a small diameter straight tube 13. A mandrel 2 is constituted of a main body part 21 which is formed to the outside diameter smaller than the inside diameter of the small diameter straight tube part 13 in the worked hole of the die 1 by roughly more than two times the wall thickness of the worked tube, and a shaft part 22 having the diameter smaller than that of the main body part 21, and provided in a relatively movable manner in the axial direction in the worked hole of the die 1.