A method for producing a Ni-based alloy, comprising: a casting step of casting a liquid alloy to produce a Ni-based alloy starting material, which has a chemical composition consisting of: in mass%, C: 0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P: 0.015% or less, S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%, one or more elements selected from the group consisting of Nb and Ta: 3.150 to 4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to 5.00%, N: 0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to 0.50%, one or more elements selected from the group consisting of Ca, Nd, and B: 0 to 0.5000%, and the balance being Ni and impurities; and a segregation reducing step of performing, on the Ni-based alloy starting material produced by the casting step, heat treatment, or the heat treatment and, after the heat treatment, complex treatment including hot working and heat treatment after the hot working, to satisfy Formula, wherein the heat treatment is performed at a holding temperature of 1000 to 1300°C: [Expression 1] V R − 0.294 ≤ 1.27 × 10 3 ∑ n = 1 N 1 − Rd n − 1 100 − 1 ⋅ exp − 2.89 × 10 4 T n + 273 ⋅ t n where, each symbol in Formula is as follows: V R: Solidification cooling rate expressed in °C/min of the liquid alloy in the casting step, as obtained from dendrite secondary arm spacing D II described in the description, T n: Holding temperature expressed in °C in the n-th heat treatment, t n: Holding time expressed in hours at the holding temperature in the n-th heat treatment, Rd n-1: Cumulative area reduction ratio expressed in % of the Ni-based alloy starting material before the n-th heat treatment, and N: Total number of the heat treatment. a casting step of casting a liquid alloy to produce a Ni-based alloy starting material, which has a chemical composition consisting of: in mass%, C: 0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P: 0.015% or less, S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%, one or more elements selected from the group consisting of Nb and Ta: 3.150 to 4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to 5.00%, N: 0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to 0.50%, one or more elements selected from the group consisting of Ca, Nd, and B: 0 to 0.5000%, and the balance being Ni and impurities; and a segregation reducing step of performing, on the Ni-based alloy starting material produced by the casting step, heat treatment, or the heat treatment and, after the heat treatment, complex treatment including hot working and heat treatment after the hot working, to satisfy Formula, wherein the heat treatment is performed at a holding temperature of 1000 to 1300°C: [Expression 1] V R − 0.294 ≤ 1.27 × 10 3 ∑ n = 1 N 1 − Rd n − 1 100 − 1 ⋅ exp − 2.89 × 10 4 T n + 273 ⋅ t n where, each symbol in Formula is as follows: V R: Solidification cooling rate expressed in °C/min of the liquid alloy in the casting step, as obtained from dendrite secondary arm spacing D II described in the description, T n: Holding temperature expressed in °C in the n-th heat treatment, t n: Holding time expressed in hours at the holding temperature in the n-th heat treatment, Rd n-1: Cumulative area reduction ratio expressed in % of the Ni-based alloy starting material before the n-th heat treatment, and N: Total number of the heat treatment. V R: Solidification cooling rate expressed in °C/min of the liquid alloy in the casting step, as obtained from dendrite secondary arm spacing D II described in the description, T n: Holding temperature expressed in °C in the n-th heat treatment, t n: Holding time expressed in hours at the holding temperature in the n-th heat treatment, Rd n-1: Cumulative area reduction ratio expressed in % of the Ni-based alloy starting material before the n-th heat treatment, and N: Total number of the heat treatment.