An Fe—Ni alloy having high strength and low thermal expansion, consisting essentially of, by mass percent, 27 to 47% of Ni, not more than 22% of Co, 0.005 to 0.1% of Nb, less than 0.01% of C, 0.002 to 0.02% of N, and the balance of Fe and inevitable impurities, wherein the following equation is fulfilled:
- 0.000013≦(% Nb)×(% N)≦0.002,
- wherein the maximum grain size of compounds primarily comprising Nb and nitrogen and other compounds primarily comprising Nb and carbon is less than 0.5 μm, which can be observed in a section of the Fe—Ni alloy, wherein the total number of the compounds primarily comprising Nb and nitrogen and other compounds primarily comprising Nb and carbon is not less than 50,000/mm2, which is determined by counting the total number of the niobium compounds observed in said section of alloy and converting the number into number per 1 mm2.