To provide a device for contracting the diameter of a metal tube capable of preventing occurrence of chatter marks on a surface of the metal tube and suppressing any increase in wall thickness, and a method for contracting the diameter of the metal tube using the device.

In a contracting device 1 of the diameter of a metal tube 3 having at least three stages of dies UD and LD, the inside diameter D1 of the die on the first stage satisfies inequality of D1/Dob ≥ 0.90, where Dn denotes the inside diameter of the die on the n-th stage from the upstream side of the dies UD and LD, and Dob denotes the outside diameter of the metal tube 3 before the metal tube passes the die on the first stage. The inside diameter Dn of the dies on the second and subsequent stages satisfies expression Dn/D(n-1) ≥ 0.80, and the inside diameter Dn of each of the continuous dies on the third stage satisfies expression D(n-1)/D(n-2)-Dn/D(n-1)≤0.05.