An electric power generating device for an aircraft, which is driven by an output of an aircraft engine, comprises an input shaft to which a driving force of the engine is transmitted; a transmission arranged with the input shaft; an electric power generator arranged with the input shaft and the transmission and driven by an output of the transmission; a driving force transmission mechanism disposed on a first end side of the input shaft in an axial direction thereof, the driving force transmission mechanism being configured to transmit the output from the transmission to the electric power generator; and a casing including a mounting section on the first end side of the input shaft in the axial direction, wherein the transmission includes an input section provided on a second end side of the input shaft in the axial direction, the input section being configured to receive as an input a rotational driving force from the input shaft, and an output section provided on the first end side of the input shaft in the axial direction, the output section being configured to output to the driving force transmission mechanism the rotational driving force whose speed has been changed, and wherein the driving force transmission mechanism has an inner space and the input shaft is inserted into the inner space.