To provide an environmental control system of a low action performance coefficient by providing a means to shield a closed loop to discharge working fluid in the middle of or after a cooling passage to the circumference, and introduce other working fluid into the loop in a second operation mode.

A cooling turbine 14 supplies cooling adjusting air for cooling an avionics load 18, a compressor 16 rises gas pressure and temperature in a loop, and heat extracted from the avionics load 18 and net power by the compressor 16 are effectively eliminated from air in the loop at an intercooler 20. As a valve 36 is moved to the position of a second mode, loop air emitted from the avionics heat load 18 is emitted to open air, and engine/APU bleed air is supplied from a pressure regulation valve 3 to the loop. When the valve 36 is at the second mode position, therefore, the loop operates as a closed loop, where cooling turbine compressor assemblies 14, 16 are operated in parallel to an output turbine 24.