To provide a method for enabling manufacture of an extremely wall- thick electric resistance welded tube minimized in thickness deviation.

In a manufacturing method of the extremely wall-thick electric resistance welded tube which is made by executing high frequency welding to the electric resistance welded tube after a steel strip having a prescribed width is rolled and formed and executing thickening by reducing the diameter by thrusting while inserting a plug to bring into contact with the inside of the tube in a rotary forging machine having a die, the taper angle (half angle) of the die is taken as 3-6° and the plug having the taper angle (half angle) reduced by 1-5° than the taper angle (half angle) of the die is used.