An exhaust system for a steam turbine that guides exhaust gas used to drive a high pressure turbine or an intermediate turbine to a downstream turbine via an exhaust duct (13, 23) when in use, the system comprising: an exhaust hood inner casing enclosing a turbine rotor; an exhaust hood outer casing surrounding the exhaust hood inner casing to define an exhaust hood (12, 22) therebetween; and an annular flow guide installed downstream of last stage rotor blades which are fixed to the turbine rotor, the annular flow guide being installed continuously with an outer circumference of the exhaust hood inner casing; wherein the flow guide includes a downstream flow guide portion at the side of the exhaust duct (13, 23) when in use and an upstream flow guide portion at the side opposite to the exhaust duct (13, 23) when in use, the two portions being formed so that the downstream flow guide portion has a greater length than the upstream flow guide portion; wherein, when an imaginary line is drawn radially from the center of the rotor on a cross-section perpendicular to a rotor axis, the distance between a root portion of the flow guide and a leading end of the same is defined as a first distance (a), and the distance between the root portion of the flow guide and an inner wall surface of the exhaust hood outer casing is defined as a second distance (b), the ratio of the first distance (a) to the second distance (b) is defined as a flow guide occupancy ratio, and the flow guide is formed so that the downstream flow guide portion has a greater flow guide occupancy ratio than the upstream flow guide portion; characterized in that the flow guide occupancy ratio of the downstream flow guide portion is between 0.6 and 0.7 inclusive, and the flow guide occupancy ratio of the upstream flow guide portion is between 0.3 and 0.6 inclusive. an exhaust hood inner casing enclosing a turbine rotor; an exhaust hood outer casing surrounding the exhaust hood inner casing to define an exhaust hood (12, 22) therebetween; and an annular flow guide installed downstream of last stage rotor blades which are fixed to the turbine rotor, the annular flow guide being installed continuously with an outer circumference of the exhaust hood inner casing; wherein the flow guide includes a downstream flow guide portion at the side of the exhaust duct (13, 23) when in use and an upstream flow guide portion at the side opposite to the exhaust duct (13, 23) when in use, the two portions being formed so that the downstream flow guide portion has a greater length than the upstream flow guide portion; wherein, when an imaginary line is drawn radially from the center of the rotor on a cross-section perpendicular to a rotor axis, the distance between a root portion of the flow guide and a leading end of the same is defined as a first distance (a), and the distance between the root portion of the flow guide and an inner wall surface of the exhaust hood outer casing is defined as a second distance (b), the ratio of the first distance (a) to the second distance (b) is defined as a flow guide occupancy ratio, and the flow guide is formed so that the downstream flow guide portion has a greater flow guide occupancy ratio than the upstream flow guide portion; characterized in that the flow guide occupancy ratio of the downstream flow guide portion is between 0.6 and 0.7 inclusive, and the flow guide occupancy ratio of the upstream flow guide portion is between 0.3 and 0.6 inclusive.