A steam turbine, comprising: a rotor shaft which includes a shaft core portion which rotates about an axis (Ar) and disk portions which are fixed to the shaft core portion and expands toward a radially outer side in the shaft core portion; and a plurality of rotor blades which are fixed to outer peripheries of the disk portions and are disposed in a circumferential direction of the shaft core portion, wherein each rotor blade includes: a blade body which extends in the radial direction, a platform which is provided on the radially inner side of the blade body, and a blade root which is provided on the radially inner side of the platform and is fitted into a blade groove formed in the disk portion, wherein a first surface which faces toward a first direction including a directional component toward a radially inner side of the shaft core portion is formed on an engaging protrusion portion which protrudes in the circumferential direction from the blade root and engages with an engaging recessed portion formed in the blade groove, a second surface which faces toward a second direction including a directional component toward the radially outer side and faces the first surface is formed on each of the disk portions, characterized in that, a first recessed portion is provided on the first surface, a second recessed portion is provided on the second surface, and a balance hole portion to decrease a pressure difference between a first side and a second side of the disk portion in an axial direction is formed of the first recessed portion and the second recessed portion. a rotor shaft which includes a shaft core portion which rotates about an axis (Ar) and disk portions which are fixed to the shaft core portion and expands toward a radially outer side in the shaft core portion; and a plurality of rotor blades which are fixed to outer peripheries of the disk portions and are disposed in a circumferential direction of the shaft core portion, wherein each rotor blade includes: a blade body which extends in the radial direction, a platform which is provided on the radially inner side of the blade body, and a blade root which is provided on the radially inner side of the platform and is fitted into a blade groove formed in the disk portion, wherein a first surface which faces toward a first direction including a directional component toward a radially inner side of the shaft core portion is formed on an engaging protrusion portion which protrudes in the circumferential direction from the blade root and engages with an engaging recessed portion formed in the blade groove, a second surface which faces toward a second direction including a directional component toward the radially outer side and faces the first surface is formed on each of the disk portions, characterized in that, a first recessed portion is provided on the first surface, a second recessed portion is provided on the second surface, and a balance hole portion to decrease a pressure difference between a first side and a second side of the disk portion in an axial direction is formed of the first recessed portion and the second recessed portion. a blade body which extends in the radial direction, a platform which is provided on the radially inner side of the blade body, and a blade root which is provided on the radially inner side of the platform and is fitted into a blade groove formed in the disk portion, wherein a first surface which faces toward a first direction including a directional component toward a radially inner side of the shaft core portion is formed on an engaging protrusion portion which protrudes in the circumferential direction from the blade root and engages with an engaging recessed portion formed in the blade groove, a second surface which faces toward a second direction including a directional component toward the radially outer side and faces the first surface is formed on each of the disk portions, characterized in that, a first recessed portion is provided on the first surface, a second recessed portion is provided on the second surface, and a balance hole portion to decrease a pressure difference between a first side and a second side of the disk portion in an axial direction is formed of the first recessed portion and the second recessed portion.