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 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, wherein each of the plurality of stepped parts 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 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, wherein each of the plurality of stepped parts 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.