To provide a method for continuously casting a large diameter cast billet with which crack in the axial center part of the cast billet can be reduced, and a method for making a pipe with which even in the case of piercing-rolling the round cast billet obtained with the above method as it is, the inner surface flaw in the pipe can be reduced.

(1) In the method for continuously casting the round cast billet having over 300 mm diameter on the cross section of the cast billet, using molten steel having ≤0.1% C, the round cast billet is cast while making the whole zone at least within 60 mm diameter of the center part of the cross section in the cast billet to an equiaxial crystal structure and also, while performing slow-cooling at ≤10°C/min cooling speed on the surface of the cast billet. (2) In the round cast billet cast with the continuous casting method above (1), the crack in the axial center part developed at the center part of the cast billet exists in the zone within 15 mm radius from the center in the cross section of the cast billet. (3) The method for making the seamless pipe is performed by piercing without applying a blooming to the round billet above (2).