A steam turbine plant of one embodiment includes a boiler to change water into steam, a high pressure turbine including plural stages of rotor and stator vanes and to be driven by the steam from the boiler, a reheater to heat the steam from the high pressure turbine, a reheat turbine including plural stages of rotor and stator vanes and to be driven by the steam from the reheater, a condenser to change the steam from the reheat turbine into water, a collector to collect water from, for example, the steam existing upstream of an inlet of the final-stage rotor vane in the high pressure turbine, and a path to cause collected matter in the collector to flow into, for example, the steam between an outlet of the final-stage rotor vane of the high pressure turbine and an inlet of the final-stage rotor vane of the reheat turbine.