a bleed air passage fluidly connected to a bleed air port of a gas turbine engine;

an auxiliary compressor structure including a compressor inlet and a compressor outlet, the compressor inlet being fluidly connected to the bleed air passage;

a supply passage which is fluidly connected to the compressor outlet and through which compressed air from the auxiliary compressor structure is supplied to an air cycle machine;

an accessory gear box which houses a power transmitting structure and to which rotational driving force of the gas turbine engine is input, the power transmitting structure including rotating shafts and gears; and

a transmission structure that changes speed a rotational driving force output from the accessory gear box and rotates the auxiliary compressor structure, wherein:

the transmission structure and the auxiliary compressor structure are fixed to the accessory gear box; and

the accessory gear box is fixed to the gas turbine engine;

the rotating shafts include a first rotating shaft, a second rotating shaft, and a third rotating shaft;

the power transmitting structure includes a first power transmitting structure and a second power transmitting structure mechanically separated from the first power transmitting structure;

the first power transmitting structure includes the first rotating shaft directly coupled to an input shaft of the transmission structure;

the second power transmitting structure includes the second rotating shaft directly coupled to an output shaft of the transmission structure and the third rotating shaft that operates in association with the second rotating shaft and is directly coupled to the driven shaft of the auxiliary compressor structure; and

the third rotating shaft is located between the first rotating shaft and the second rotating shaft.