To provide a motive power generating device capable of simplifying a structure and control of the device, easy in maneuvering and attitude control of a floating body, and capable of efficiently generating motive power.

This motive power generating device includes the floating body 11 floating in the air by wind power, at least two ropes 17 and 18 having one end side connected to the floating body 11, rotary bodies 23 and 24 connected to the other end side of the ropes 17 and 18 and generating the motive power by rotating, and a tension adjusting means 12 for adjusting tension of the ropes 17 and 18 by controlling force receiving from wind to the floating body 11 by relatively moving the two ropes 17 and 18 in the length direction. The tension adjusting means 12 increases the tension of the ropes 17 and 18 to rotate the rotary bodies 23 and 24 by the tension of the ropes 17 and 18 in a partial rotation range R1 of the rotary bodies 23 and 24, and reduces the tension of the ropes 17 and 18 to rotate the rotary bodies 23 and 24 by inertial force in the other rotation range R2 of the rotary bodies 23 and 24.