Provided is a method for manufacturing a round billet of Ni-containing high alloy including steps of continuously casting a molten steel by using a casting mold, thereby obtaining a rectangular billet, and shaping the rectangular billet into round billet by forging/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 perpendicular to the casting direction of rectangular billet is 1.0 to 2.0, the chemical composition of molten steel is, by mass %: C: 0.005 to 0.250%, Si: 0.05 to 2.00%, Mn: 0.05 to 3.00%, P: 0.04% or less, S: 0.004% or less, Cu: 0.01 to 3.00%, Cr: 10 to 35%, Ni: 10 to 80%, Mo: 1.5 to 10.0%, Al: 0.001 to 0.300%, N: 0.001 to 0.300%, W: 0.00 to 6.00%, Ti: 0.00 to 2.00%, and the balance: Fe and impurities.