A steam turbine (10) of an embodiment has seal rings (60) between an inlet sleeve (40) for introducing steam and an inner casing (20) and an outer casing (21) into which the inlet sleeve (40) is inserted. The seal rings (60) have high-temperature side seal rings (70) which are disposed their inner circumferences contacted to the outer circumference of the inlet sleeve (40), and low-temperature side seal rings (80) which are formed to have inner and outer diameters larger than those of the high-temperature side seal rings (70) and disposed with their outer circumferences contacted to the inner casing (20) or the outer casing (21). A thermal barrier layer (90) is disposed between the inner circumferences of the high-temperature side seal rings (70) and the outer circumference of the inlet sleeve (40) and between the high-temperature side seal rings (70) and the low-temperature side seal rings (80).