A steam turbine of an embodiment includes: a nozzle box having a circular ring-shaped steam passage into which steam is led from a steam inlet pipe and having, on a downstream portion of the steam passage, a plurality of first-stage nozzles in a circumferential direction; and a second-stage nozzle side structure combined with the nozzle box and having a plurality of at least second-stage nozzles in a circumferential direction. Further, in the respective nozzle box and second-stage nozzle side structure, there are provided at least a pair of abutment surfaces for sealing which are not parallel to a turbine rotor axis and abutting each other. Furthermore, a seal member is provided in a part of a gap between the nozzle box and the second-stage nozzle side structure, the gap being formed by combining the nozzle box and the second-stage nozzle side structure.