A method for manufacturing a round billet of Ni-containing high alloy, the method comprising the steps of: continuously casting a molten steel by means of a casting mold, thereby obtaining a rectangular billet; and shaping the rectangular billet into a round billet by forging or rolling, wherein the casting mold has a shape such that a w/h ratio between long side length (w) and short side length (h) of a cross section of the rectangular billet is 1.0 to 2.0, and the cross section is perpendicular to the casting direction of the rectangular billet; wherein the shaping step includes soaking the rectangular billet at 1200 to 1300°C before the forging or rolling; and the soaking time of the rectangular billet in the soaking step is no less than 9 hours where the shaping is carried out by forging, and is no less than 24 hours where the shaping is carried out by rolling; the chemical composition of the molten steel is: C: 0.005 to 0.250% by mass, Si: 0.05 to 2.00% by mass, Mn: 0.05 to 3.00% by mass, P: no more than 0.04% by mass, S: no more than 0.004% by mass, Cu: 0.01 to 3.00% by mass, Cr: 10 to 35% by mass, Ni: 10 to 80% by mass, Mo: 1.5 to 10.0% by mass, Al: 0.001 to 0.300% by mass, N: 0.001 to 0.300% by mass, W: 0.00 to 6.00% by mass, Ti: 0.00 to 2.00% by mass, and the balance: Fe and impurities; a Mo segregation degree of the round billet is no more than 0.20; and the Mo segregation degree is a ratio ((c Mo '-c Mo) /c Mo) of a difference (c Mo '-c Mo) between Mo concentration at the axial center of the round billet (c Mo ') and Mo concentration in the molten steel (c Mo) to Mo concentration in the molten steel (c Mo). continuously casting a molten steel by means of a casting mold, thereby obtaining a rectangular billet; and shaping the rectangular billet into a round billet by forging or rolling, wherein the casting mold has a shape such that a w/h ratio between long side length (w) and short side length (h) of a cross section of the rectangular billet is 1.0 to 2.0, and the cross section is perpendicular to the casting direction of the rectangular billet; wherein the shaping step includes soaking the rectangular billet at 1200 to 1300°C before the forging or rolling; and the soaking time of the rectangular billet in the soaking step is no less than 9 hours where the shaping is carried out by forging, and is no less than 24 hours where the shaping is carried out by rolling; the chemical composition of the molten steel is: C: 0.005 to 0.250% by mass, Si: 0.05 to 2.00% by mass, Mn: 0.05 to 3.00% by mass, P: no more than 0.04% by mass, S: no more than 0.004% by mass, Cu: 0.01 to 3.00% by mass, Cr: 10 to 35% by mass, Ni: 10 to 80% by mass, Mo: 1.5 to 10.0% by mass, Al: 0.001 to 0.300% by mass, N: 0.001 to 0.300% by mass, W: 0.00 to 6.00% by mass, Ti: 0.00 to 2.00% by mass, and the balance: Fe and impurities; C: 0.005 to 0.250% by mass, Si: 0.05 to 2.00% by mass, Mn: 0.05 to 3.00% by mass, P: no more than 0.04% by mass, S: no more than 0.004% by mass, Cu: 0.01 to 3.00% by mass, Cr: 10 to 35% by mass, Ni: 10 to 80% by mass, Mo: 1.5 to 10.0% by mass, Al: 0.001 to 0.300% by mass, N: 0.001 to 0.300% by mass, W: 0.00 to 6.00% by mass, Ti: 0.00 to 2.00% by mass, and the balance: Fe and impurities; a Mo segregation degree of the round billet is no more than 0.20; and the Mo segregation degree is a ratio ((c Mo '-c Mo) /c Mo) of a difference (c Mo '-c Mo) between Mo concentration at the axial center of the round billet (c Mo ') and Mo concentration in the molten steel (c Mo) to Mo concentration in the molten steel (c Mo).