A steam turbine plant comprising: a boiler configured to change water into steam; an upstream turbine including plural stages of rotor vanes and plural stages of stator vanes, and configured to be driven by the steam from the boiler; a downstream turbine including plural stages of rotor vanes and plural stages of stator vanes, and configured to be driven by the steam from the upstream turbine; and a condenser configured to change the steam exhausted from the downstream turbine into water; the plant being characterised by further comprising: a collector configured to collect water from the steam which exists upstream of an inlet of the final-stage rotor vane in the upstream turbine; a gas-liquid separator configured to separate the collected matter collected in the collector or resultant matter changed from the collected matter into a gas and a liquid; and a collected matter path on which the gas-liquid separator is disposed, the collected matter path being configured to cause the separated gas to flow into: the steam between an outlet of the final-stage rotor vane of the upstream turbine and an inlet of the final-stage rotor vane of the downstream turbine, or the steam between a collection place of the collected matter and the inlet of the final-stage rotor vane in the upstream turbine. a boiler configured to change water into steam; an upstream turbine including plural stages of rotor vanes and plural stages of stator vanes, and configured to be driven by the steam from the boiler; a downstream turbine including plural stages of rotor vanes and plural stages of stator vanes, and configured to be driven by the steam from the upstream turbine; and a condenser configured to change the steam exhausted from the downstream turbine into water; the plant being characterised by further comprising: a collector configured to collect water from the steam which exists upstream of an inlet of the final-stage rotor vane in the upstream turbine; a gas-liquid separator configured to separate the collected matter collected in the collector or resultant matter changed from the collected matter into a gas and a liquid; and a collected matter path on which the gas-liquid separator is disposed, the collected matter path being configured to cause the separated gas to flow into: the steam between an outlet of the final-stage rotor vane of the upstream turbine and an inlet of the final-stage rotor vane of the downstream turbine, or the steam between a collection place of the collected matter and the inlet of the final-stage rotor vane in the upstream turbine. a collector configured to collect water from the steam which exists upstream of an inlet of the final-stage rotor vane in the upstream turbine; a gas-liquid separator configured to separate the collected matter collected in the collector or resultant matter changed from the collected matter into a gas and a liquid; and a collected matter path on which the gas-liquid separator is disposed, the collected matter path being configured to cause the separated gas to flow into: the steam between an outlet of the final-stage rotor vane of the upstream turbine and an inlet of the final-stage rotor vane of the downstream turbine, or the steam between a collection place of the collected matter and the inlet of the final-stage rotor vane in the upstream turbine. the steam between an outlet of the final-stage rotor vane of the upstream turbine and an inlet of the final-stage rotor vane of the downstream turbine, or the steam between a collection place of the collected matter and the inlet of the final-stage rotor vane in the upstream turbine.