PURPOSE: To smooth both end faces of a blank pipe after constricting its diameter, to increase wall thickness of a crotch and a branch, and to manufacture a durable, lightweight tee joint in good yields, by previously giving a longitudinal compression to the branching side of an elliptical pipe before constricting the diameter.

CONSTITUTION: After heating the elliptical pipe 1, the branching side is compressed from both sides by using the press 7, etc., to the extent possible to absorb the over- filling caused by the constriction of diameter. Next, an open hole is bored at the scheduled branching part by the drill. While this elliptical pipe 1 is being compressed in the longitudinal direction by the press 2, a part of wall thickness is drawn in the direction of compression by using the plug 3 and the die 4. According to the present method, the over-filling at the pipe end is prevented and the wall thickness of the crotch and the branch is increased, so that the mechanical strength is improved. Moreover, since the height of the branch is increased as a result of increase in the wall thickness of the branch, the wall thickness of the original pipe thinned and the weight of the product tee joint is lightened.