PURPOSE: To keep the air delivery pressure of a compressor in a specified value and decide an optimum nozzle opening by feedback-controlling the air delivery pressure of a compressor by operating the opening of variable nozzle with a turbocompressor.

CONSTITUTION: The air delivery rate of a compressor is proportional to turbine power if air delivery pressure is constant. The turbine power is proportional to the square root of gas temperature in the inlet of the turbine and to the opening of a variable nozzle 2c if the gas pressure in the inlet of the turbine is constant. If the air delivery pressure of the compressor falls from a specified value, the air delivery rate of the compressor becomes smaller than the air flow rate supplied to a load 26. By increasing the opening of the variable nozzle 2c, turbine power is increased and the air delivery rate of the compressor is increased, and the air delivery pressure of the compressor is increased near a specified value. By this operation, a controller 27 operates the opening of the nozzle based on the data of the air delivery pressure of the compressor detected by a pressure detector 4 to feedback-control the air delivery pressure of the compressor in a specified value.