A steam turbine, comprising: a turbine rotor penetratingly provided in a casing, and having a plurality of stages of rotor disks projected to an outside in a radial direction along a circumferential direction, in a turbine rotor axial direction; rotor blade cascades each configured by implanting a plurality of rotor blades in the rotor disk in the circumferential direction; stationary blade cascades each configured by supporting a plurality of stationary blades in the circumferential direction between a diaphragm outer ring and a diaphragm inner ring provided on an inside of the casing, and each arranged alternately with the rotor blade cascade in the turbine rotor axial direction, characterized in that a steam passage is formed on the rotor disk of a turbine stage and configured to lead a leakage steam from an upstream side to a downstream side of the rotor disk, the turbine stage including the rotor blades each having a blade height equal to or more than a blade height at which a loss generated when the leakage steam flown downstream between the diaphragm inner ring and the turbine rotor jets into a main steam from a portion between the diaphragm inner ring and the rotor blades and a benefit brought by increasing the blade height of each of the rotor blades in accordance with an increase in a flow rate of the main steam due to the jet of the leakage steam into the main steam are cancelled, the turbine stage being one of a plurality of turbine stages each configured by the stationary blade cascade and the rotor blade cascade arranged at an immediately downstream side of the stationary blade cascade. a turbine rotor penetratingly provided in a casing, and having a plurality of stages of rotor disks projected to an outside in a radial direction along a circumferential direction, in a turbine rotor axial direction; rotor blade cascades each configured by implanting a plurality of rotor blades in the rotor disk in the circumferential direction; stationary blade cascades each configured by supporting a plurality of stationary blades in the circumferential direction between a diaphragm outer ring and a diaphragm inner ring provided on an inside of the casing, and each arranged alternately with the rotor blade cascade in the turbine rotor axial direction, characterized in that a steam passage is formed on the rotor disk of a turbine stage and configured to lead a leakage steam from an upstream side to a downstream side of the rotor disk, the turbine stage including the rotor blades each having a blade height equal to or more than a blade height at which a loss generated when the leakage steam flown downstream between the diaphragm inner ring and the turbine rotor jets into a main steam from a portion between the diaphragm inner ring and the rotor blades and a benefit brought by increasing the blade height of each of the rotor blades in accordance with an increase in a flow rate of the main steam due to the jet of the leakage steam into the main steam are cancelled, the turbine stage being one of a plurality of turbine stages each configured by the stationary blade cascade and the rotor blade cascade arranged at an immediately downstream side of the stationary blade cascade.