To provide a method for machining an inner grooved tube, which can easily apply twist to a metal tube and can increase a lead angle of the inner groove.

In the method for machining an inner grooved tube, a floating plug 2 and a grooved plug 4 connected to the floating plug 2 through a connecting rod 3 are arranged within a metal tube 1 withdrawn from a metal tube coil 18. The metal tube 1 is passed through a drawing die 9 to fasten the floating plug 2 to the drawing die 9. At the position of the grooved plug 4, the metal tube 1 is pressed on the grooved plug 4 to form a plurality of grooves 8 on the inner surface of the metal tube 1. The metal tube 1 is withdrawn in an axial direction from the metal tube coil 18, and, further, the metal tube coil 18 is rotated around the axis so that the metal tube 1 is rotated in the same direction as the direction of rotation of the grooved plug 4.