A stage structure of an axial turbine includes a stationary inner ring, a stationary outer wall, a row of stationary blades mounted on the stationary inner ring and outer wall, a row of moving blades, and a shroud ring mounted on the tips of the moving blades. An annular solid substance, disposed immediately downstream of an axial gap formed between an axial end of the shroud ring and surface of the outer wall axially facing the axial end of the shroud ring, reduces an expansion space provided immediately downstream of the axial gap, whereby circulation of an ejection flow from a main stream through the axial gap is reduced so that the turbine stage efficiency is improved. The annular solid substance may be a ring fixed to the stationary outer wall, a protrusion or a cylinder formed by a portion of the outer wall.