To easily and surely form a small-diameter part, which is made eccentric, integrally by applying simple cold plastic working to a metallic tube material.

A method for working a metal tube is used for forming a tapered part 10 and a small-diameter part 9 that is continuously and non-coaxially connected to the part 10 in a metallic tube raw material 7. A main body 8 and the small-diameter part 9 of the metallic tube raw material 7, in which the tapered part 10 and the small-diameter part 9 continuously connected to the part 10 are formed, are firmly held by grasping members 1, 2 respectively. The non-coaxial small-diameter part 9 is formed by relatively moving the grasping members 1, 2 in a direction non-perpendicular to the tube axis.