PURPOSE: To produce a steel tube reduced in yield ratio from the consideration of safety to large earthquake and usable in the fields of building, bridge, etc.

CONSTITUTION: After a strain of 3-10% is applied at 900-750°C to a tube stock in the course of manufacture by the ordinary rolling system, the tube stock is subjected, without cooling down to ≤650°C, to reheating up to 900-1150°C, to sizing, and to air cooling. Then, the resulting tube is heated and held to and at a temp. of Ac1 transformation point +(10-130°C) to undergo preferential α-γ transformation and cooled rapidly or further subjected to tempering treatment, by which the high strength seamless steel tube reduced in yield ratio can be produced.