PURPOSE: To improve dimensional accuracy heating the corresponding part of a billet where the amt. of the material flow into a die hole is small to a temp. higher than in the other parts in hot extrusion of an outside surface deformed pipe.

CONSTITUTION: The billet to be supplied into the die 2 is extruded by heating the corresponding part thereof where the amt. of the material flow into the die hole 2a is small to the temp. higher than in the part where the amt. of the material flow into the die hole 2a is large. Further, the die larger in the approach angle α of the corresponding part where the amt. of the material flow into the die hole 2a is small than the corresponding part where the amt. of the material flow is large is used at need. The dimensional accuracy of the outside surface deformed pipe is improved in this way and the yield of production is improved.