PURPOSE: To manufacture a base stock for high alloy seamless steel tube with high yield by specifying the cross section of the base stock and its rolling quantity when a round billet for seamless steel tube is manufactured from a cast bloom containing Cr of the critical weight percentage.

CONSTITUTION: At the time of manufacturing the round billet for seamless steel tube from a confinuously cast bloom of iron alloy containing Cr of 0.5-40wt.%, the continuously cast bloom with a rectangular sectional shape of which the ratio of the long side to the short side is 1.5-2.5 is used for a base stock and the round billet for seamless steel tube is manufactured by applying rolling work at the draft of ≥9.0.