PURPOSE: To prevent foreign matter in exhaust gas from infiltrating and accumulating in a gap between a mobile passage wall and a casing and prevent the mobile passage wall from being stuck nonslidably.

CONSTITUTION: A mobile passage wall 5 reciprocatable in the exhaust gas passage of the moving blade inlet of a radial turbine (inlet side passage of a nozzle blade 1) 3 is movably provided in a gas inlet casing 8. Multiple seal plates 21 covering the exhaust gas passage opening section 13a of a gap 13 generated between the mobile passage wall 5 and the gas inlet casing 8 are provided on the opening section 13a. The seal plates 21 are circularly arranged to have overlapped portions in the peripheral direction, they are fixed to the gas inlet casing 8 at one end, and they are invariably brought into contact with the mobile passage wall 5 by flexible force at the other end when the mobile passage wall 5 is moved in the axial direction. Foreign matter in exhaust gas 17 are prevented from infiltrating into the gap 13 from the opening 13a.