To provide a screw-forming method for the container mouth capable of eliminating fluctuation of diametrical dimensions of the ridge and the bottom of the screw.

In the screw-forming method for the container mouth 2 in which the container mouth 2 is put in between the internal pieces 4 and the external pieces 5 and the screw is formed on the container mouth 2, the bottom 8 of the screw 6 is put without clearance between the recesses 7 of the internal pieces 4 and the protrusions 9 of the external pieces 5 when forming the bottom of the screw 6 to form the screw, as the feature of the screw-forming method.