PROBLEM TO BE SOLVED: To provide a Ni based alloy having the state of a passive film and capable of withdrawing excellent corrosion resistance, and a manufacturing method of controlling O, S and C in the alloy to be low in order to stabilize the corrosion resistance.SOLUTION: In the Ni based alloy, a ratio of a Cr concentration (at.%) to a Ni concentration (at.%) in a passive film part formed on the surface of the alloy is 0.2 or more, and a critical crevice corrosion generation temperature (CCT) in the ASTM G48 method D test is 40°C or more. The Ni based alloy preferably consists of C:0.002-0.05%, S:0.0001-0.005%, Cr:10-25%, Mo:5-20%, O:0.0001-0.005%, Al:0.01-0.5%, Fe:0.5-10%, the remainder Ni and inevitable impurities. The method for manufacturing the Ni based alloy comprises: melting a raw material by an electric furnace; decarbonizing in AOD and/or VOD, Cr-reducing and desulfurizing the molten material; and preparing the molten alloy in the above-mentioned component range.
