Radial turbine and method of cooling nozzle of the same

A turbine nozzle that, among components constructing a turbine, reaches particularly high temperature is efficiently cooled with a relatively simple structure. A double casing structure in which a turbine casing is provided outside a turbine shell is formed. The turbine casing functions as a flow path for compressed air (20-> 21) before combustion. The turbine shell covers a turbine nozzle and a radial turbine impeller and forms flow paths for combustion gas (10->11->12->13). The compressed air before combustion flowing in the flow path, for compressed air, having air-tightness between itself and the outside air is blown to the turbine nozzle through a through-hole penetrating both wall surfaces of the turbine shell. By this, the turbine nozzle is cooled and the compressed air used to cool the turbine nozzle is made to flow toward the turbine impeller. As a result, the entire volume of compressed air taken into the turbine contributes to mechanical work for driving the turbine impeller.