A steam turbine system comprising: a steam turbine including a plurality of rotor blades that are configured to rotate around a rotating shaft with steam within a casing and are arranged at intervals in a central axis direction of the rotating shaft so as to form a plurality of stages; a first mixed steam supply pipe that is configured to supply the steam, which is supplied from a steam supply source allowing supply of the steam with fluctuating pressure, to an upstream stage (Sa) within the casing ; a second mixed steam supply pipe that is provided to branch from the first mixed steam supply pipe and is configured to supply the steam to a downstream stage (Sb) located on a downstream side with respect to the upstream stage (Sa) within the casing ; an adjusting unit that is configured to adjust a flow rate of the steam supplied from the first mixed steam supply pipe to the upstream stage (Sa) and a flow rate of the steam supplied from the second mixed steam supply pipe to the downstream stage (Sb); a detection unit that is configured to detect a differential pressure between a pressure of the steam supplied from the steam supply source and a pressure in the upstream stage (Sa); a control unit that is configured to control the adjusting unit on the basis of the differential pressure detected by the detection unit ; characterized in that a steam supply line (L1) is connected to a steam inlet of the steam turbine and is configured to supply main steam from a boiler to the steam inlet , wherein the steam turbine has a plurality of the stepped parts (S), wherein each of the plurality of stepped parts (S) is composed of the rotor blade and a stator blade adjacent to an upstream side of the rotor blade , wherein the steam inlet is located on the uppermost stream side within the casing , and wherein the upstream stage (Sa) is located on a downstream side via at least one stepped part with respect to a space connected to the steam inlet . a steam turbine including a plurality of rotor blades that are configured to rotate around a rotating shaft with steam within a casing and are arranged at intervals in a central axis direction of the rotating shaft so as to form a plurality of stages; a first mixed steam supply pipe that is configured to supply the steam, which is supplied from a steam supply source allowing supply of the steam with fluctuating pressure, to an upstream stage (Sa) within the casing ; a second mixed steam supply pipe that is provided to branch from the first mixed steam supply pipe and is configured to supply the steam to a downstream stage (Sb) located on a downstream side with respect to the upstream stage (Sa) within the casing ; an adjusting unit that is configured to adjust a flow rate of the steam supplied from the first mixed steam supply pipe to the upstream stage (Sa) and a flow rate of the steam supplied from the second mixed steam supply pipe to the downstream stage (Sb); a detection unit that is configured to detect a differential pressure between a pressure of the steam supplied from the steam supply source and a pressure in the upstream stage (Sa); a control unit that is configured to control the adjusting unit on the basis of the differential pressure detected by the detection unit ; characterized in that a steam supply line (L1) is connected to a steam inlet of the steam turbine and is configured to supply main steam from a boiler to the steam inlet , wherein the steam turbine has a plurality of the stepped parts (S), wherein each of the plurality of stepped parts (S) is composed of the rotor blade and a stator blade adjacent to an upstream side of the rotor blade , wherein the steam inlet is located on the uppermost stream side within the casing , and wherein the upstream stage (Sa) is located on a downstream side via at least one stepped part with respect to a space connected to the steam inlet . <EOS>