An electric power generating device (1) capable of suppressing an increase of a frontal surface area of an aircraft engine includes a transmission (22) connected with a rotary shaft (9) of the engine (E), an electric power generator (34) driven by an output of the transmission (22), an input shaft (27) having a shaft axis extending in a direction crossing the rotary shaft (9) and connected with the rotary shaft (9), and a transmitting mechanism (21) connected with the input shaft (27) to drive the transmission (22) about an axis extending in a direction perpendicular to the input shaft (27). The transmission (22) and the electric power generator (34) are disposed spaced a distance from each other in a direction circumferentially of the rotary shaft (9).