Abstract: Each engine is directly coupled to an engine deceleration device composed of a traction speed change mechanisms and a planetary speed change mechanism, and is decelerated at a desired speed change ratio, and rotate and drive a collector gear and a main rotor shaft. The collector gear rotates and drives accessories and a tail rotor through a second traction speed change mechanism changing speed in inverse number of the speed change ratio of the traction speed change mechanisms. Thus, the rotating speed of the engines can be continuously decelerated at an arbitrary speed change ratio, and an engine deceleration device of small size, light weight, and low loss is realized. At the same time, a power transmission device for helicopters capable of changing the rotating speed of the main rotor at a constant engine rotating speed is realized.