The invention provides an axial flow machine that effectively reduces the unstable hydrodynamic force induced by leakage flow and thereby prevents unstable vibrations.A steam turbine comprises: a ring-shaped cover 6 connected to the outer circumferential side of a rotor blade row 4; and a ring-shaped concave section 12 provided on an inner circumferential surface 8 of a casing 1 for housing the cover 6. A narrow passage 15 is formed between an outer circumferential surface 13 of the cover 6 and a bottom surface 14 of the concave section 12. A narrow inflow passage 18 is formed between an upstream lateral surface 16 of the cover 6 and an upstream lateral surface 17 of the concave section 12. A narrow outflow passage 21 is formed between a downstream lateral surface 19 of the cover 6 and a downstream lateral surface 20 of the concave section 12. Between the narrow inflow passage 18 and the narrow passage 15 lies an expanded inflow passage 22. The expanded inflow passage 22 has a substantially uniform structure in a circumferential direction and is formed such that it is located on the more outer circumferential side than the bottom surface 20 of the concave section 12 and such that it is located upstream side in terms of the rotor's axial direction with respect to the upstream lateral surface 17 of the concave section 12.