Abstract: The invention intends to provide a material that exhibits excellent corrosion resistance to supercritical ammonia and is suitable for a supercritical ammonia reactor. An Ni-based corrosion resistant alloy includes from 15% or more to 50% or less by mass of Cr and any one or both of Mo and W, wherein a [(content of Mo)+0.5×(content of W)] is from 1.5% or more to 8.5% or less by mass, a value of 1.8×[% content of Cr]/{[% content of Mo]+0.5×[% content of W]} is from 3.0 or more to 70.0 or less and the balance is Ni and an unavoidable impurity. Preferably, content of Fe is less than 3% by mass, and content of C is less than 0.05% by mass. The alloy is used to configure a supercritical ammonia reactor or the material is coated on a surface that contacts with a supercritical ammonia fluid. The alloy exhibits excellent corrosion resistance to supercritical ammonia and a mineralizer added the supercritical ammonia.