A fluid flow machine has a main flow path 2 which is confined by a hub 3 and a casing 1 and in which at least one row of blades 5 is arranged. A gap 11 is provided on at least one blade row 5 between a blade end and a main flow path confinement, with the blade end and the main flow path confinement performing a rotary movement relative to each other. At least one reversing duct 7 is provided in the area of the blade leading edge in the main flow path confinement at a discrete circumferential position. The reversing duct 7 connects two openings 12, 13 arranged on the main flow path confinement.