A heat resistant steel for use as a material of engine valves, having a composition containing, by weight, not less than 0.01% and below 0.20% of carbon, from 0.05% to 1.0% of silicon, from 7.5% to 15.0% of manganese, from 2.0% to 20.0% in total of at least one of nickel and cobalt, from 15.0% to 25.0% of chromium, not more than 3.0% of molybdenum, above 2.0% and not more than 10.0% of tungsten, not less than 0.01% and below 0.50% of niobium, from 0.30% to 0.65% of nitrogen, not more than 0.02% of boron, and the balance incidental inclusions and iron. Cobalt content is determined to meet the condition of % Co = (Ni ± 5)%. The heat resistant steel meets the conditions of: oxidation weight loss when held at 1000°C for 100 hours being not greater than 0.15 mg/cm²/hour in atmosphere; tensile strength being not less than 196.2 N/mm²(20 kgf/mm²) at 900°C after a solution treatment at 1030 to 1070°C and a subsequent aging treatment; and creep rupture life at 900°C under stress load of 58.9 N/mm²(6 kgf/mm²) being not less than 25 hours.