Abstract: A gas turbine has a casing forming a fluid passage through which a primary luid flows, and a rotor blade disposed in the fluid passage with a clearance between distal ends of the rotor blades and the casing. The casing has a plurality of spaced blow holes extending circumferentially thereof for communicating an external secondary fluid chamber with the clearance, the blow holes being formed at an incline with respect to the wall of the casing in such a manner that a secondary fluid discharged from the blow holes is imparted with a flow component that opposes a flow of the primary fluid leaking from the clearance. The amount of the secondary fluid discharged from the blow holes and the discharge pressure are controlled to hydromechanically vary the resistance which the secondary fluid offers to the primary fluid leakage flow, thereby making it possible to mimimize the leakage flow.