PROBLEM TO BE SOLVED: To provide a free-forging processing method capable of controlling in a desired crystal grain, when fining a crystal grain of an Ni group thermostable alloy member by the pin stop effect by a δ phase.SOLUTION: After executing δ phase deposition treatment for depositing a δ phase in a needle shape in advance to an Ni group thermostable alloy material having a composition of Ni:50-55%, Cr:17-21%, Al:0.2-0.8%, Ti:0.6-1.2%, Nb:4.7-5.6%, Mo:2.8-3.3%, Co:≤1.0%, C+Si+Mn+P+S+Cu+B:≤1.1% and a residual part of Fe in mass%, a forging object material by δ phase deposition treatment is heated at 920-1025°C for 1-36hr, a deposited needle-like δ phase is partially solidly solved by causing segmentation, and initial quantity adjustment processing of the δ phase is executed for adjusting a deposition quantity of the δ phase, and afterwards, a finishing forging process is executed for repeating free forging and reheating by at least one time or more for add
