A Ni-based heat resistant alloy as pipe, plate, rod, forgings and the like consists of C≦0.15%, Si≦2%, Mn≦3%, P≦0.03%, S≦0.01%, Cr: 15% or more and less than 28%, Mo: 3 to 15%, Co: more than 5% and not more than 25%, Al: 0.2 to 2%, Ti: 0.2% to 3%, Nd: fn to 0.08%, and O≦0.4Nd, further containing, as necessary, at least one kind of Nb, W, B, Zr, Hf, Mg, Ca, Y, La, Ce, Ta, Re and Fe of specific amounts, the balance being Ni and impurities, wherein, fn=1.7×10−5d+0.05{(Al/26.98)+(Ti/47.88)+(Nb/92.91)}. In the formula, d denotes an average grain size (μm), and each element symbol denotes the content (mass %) of that element. If the alloy contains W, Mo+(W/2)≦15% holds. The alloy has improved ductility after long-term use at high temperatures, and cracking due to welding can be avoided.