PURPOSE: To increase the output of an engine under intermediate and low output zones, by opening an opening regulation valve arranged in the way of a bypass piping under said zones.

CONSTITUTION: When a high output is requested during when an engine is operating in an intermediate or low output zone, an opening regulation valve 26 is opened so as to inject fuel from an injection valve 28a and ignite said fuel thus feeding continuous combustion energy from an external combuster 28 through a bypass piping 24 to a turbine 14b. Consequently, kinetic energy being received by the turbine 14b increases thus increasing the supercharge pressure.