A Ni-based alloy comprising: a composition consisting of, by mass, 20.0% to 26.0% Cr, 4.7% to 9.4% Co, 5.0% to 16.0% Mo, 0.5% to 4.0% W, 0.3% to 1.5% Al, 0.1% to 1.0% Ti, 0.001% to 0.15% C, and optionally 0.01% to 5% Fe, optionally 0.0005% to 0.05% Ca, optionally 0.0005% to 0.05% Mg, optionally 0.001% to 0.15% a rare earth element, optionally 0.01% to 1.0% Nb, optionally 0.01% to 1.0% Ta, optionally 0.01% to 1.0% V, optionally 0.002% to 0.01% B, optionally 0.001% to 0.05% Zr, and the balance of Ni with inevitable impurities; wherein the Ni-based alloy comprises titanium nitrides, of which an estimated largest size is an area-equivalent diameter, the estimated largest size of the titanium nitrides being determined by calculating an area-equivalent diameter D which is defined as D = A 1/2 in relation to an area A of a titanium nitride with a largest size among titanium nitrides present in a measurement field of view area So of an observation of the Ni-based alloy, repeatedly performing this operation for n times corresponding to a measurement field of view number n to acquire n pieces of data of the area-equivalent diameter D, arranging the pieces of data of area-equivalent diameter D in ascending order into D 1, D 2,... D n to calculate a standardized variable y j which is defined by the following expression: y j = − ln − ln j / n + 1 wherein j denotes the number of the arranged ascending order of the data of the area-equivalent diameter D, plotting the area-equivalent diameter D and the standardized variable y j on X and Y axes of an X-Y coordinate system, respectively, to obtain a regression line y j = a × D + b (wherein a and b are constants), calculating y j from the following expression: y j = − ln − ln S S 0 + S where a cross-sectional area to be predicted S is 100 mm 2, and substituting the obtained value of y j into the regression line to obtain the estimated largest size of the titanium nitrides; wherein So equals 0.306mm 2, n equals 50, and the observation of the Ni-based alloy is at an observation magnification of 450 times for selecting the nitrides and at 1000 times for measuring the nitrides; characterized in that the estimated largest size of the titanium nitrides is between 12μm and 25μm. a composition consisting of, by mass, 20.0% to 26.0% Cr, 4.7% to 9.4% Co, 5.0% to 16.0% Mo, 0.5% to 4.0% W, 0.3% to 1.5% Al, 0.1% to 1.0% Ti, 0.001% to 0.15% C, and optionally 0.01% to 5% Fe, optionally 0.0005% to 0.05% Ca, optionally 0.0005% to 0.05% Mg, optionally 0.001% to 0.15% a rare earth element, optionally 0.01% to 1.0% Nb, optionally 0.01% to 1.0% Ta, optionally 0.01% to 1.0% V, optionally 0.002% to 0.01% B, optionally 0.001% to 0.05% Zr, and the balance of Ni with inevitable impurities; wherein the Ni-based alloy comprises titanium nitrides, of which an estimated largest size is an area-equivalent diameter, the estimated largest size of the titanium nitrides being determined by calculating an area-equivalent diameter D which is defined as D = A 1/2 in relation to an area A of a titanium nitride with a largest size among titanium nitrides present in a measurement field of view area So of an observation of the Ni-based alloy, repeatedly performing this operation for n times corresponding to a measurement field of view number n to acquire n pieces of data of the area-equivalent diameter D, arranging the pieces of data of area-equivalent diameter D in ascending order into D 1, D 2,... D n to calculate a standardized variable y j which is defined by the following expression: y j = − ln − ln j / n + 1 wherein j denotes the number of the arranged ascending order of the data of the area-equivalent diameter D, plotting the area-equivalent diameter D and the standardized variable y j on X and Y axes of an X-Y coordinate system, respectively, to obtain a regression line y j = a × D + b (wherein a and b are constants), calculating y j from the following expression: y j = − ln − ln S S 0 + S where a cross-sectional area to be predicted S is 100 mm 2, and substituting the obtained value of y j into the regression line to obtain the estimated largest size of the titanium nitrides; wherein So equals 0.306mm 2, n equals 50, and the observation of the Ni-based alloy is at an observation magnification of 450 times for selecting the nitrides and at 1000 times for measuring the nitrides; where a cross-sectional area to be predicted S is 100 mm 2, and substituting the obtained value of y j into the regression line to obtain the estimated largest size of the titanium nitrides; wherein So equals 0.306mm 2, n equals 50, and the observation of the Ni-based alloy is at an observation magnification of 450 times for selecting the nitrides and at 1000 times for measuring the nitrides; characterized in that the estimated largest size of the titanium nitrides is between 12μm and 25μm.