A bearing air passage through which bearing air is supplied toward an air bearing of an air motor and a turbine air passage through which driving air is supplied toward a turbine are provided in a housing. An annular space surrounding the air motor is provided between the housing and a cover. A bearing air branch passage connecting the annular space and the bearing air passage to each other and a turbine air branch passage for connecting the annular space and the turbine air passage to each other are provided. The air branch passages lead a part of the compressed air to the annular space and keep the cover in a heated state.