PURPOSE: To simplify a spring shaft in structure as well as to facilitate its maintenance by constituting this spring shaft connecting a planetary gear and an object to each other so as to make itself to absorb or discharge the energy, in a device which makes the object perform its uniform motion and generates a thrust in use of a planetary gear mechanism.

CONSTITUTION: A thrust generator, utilizing a planetary gear mechanism, makes an object perform its ununiform motion, thereby generating a thrust. Then, a planetary gear B and this object G both are connected to each other via a planetary shaft C and a connecting rod D. In this case, a spring shaft equivalent to a part of the planetary shaft C is formed into such a structure as making this shaft itself absorb or discharge the energy. In brief, a flange N is attached to one end of a gear shaft M via a rolling bearing P, and a weight equivalent to the object is installed in a hole O of this flange N, while a torsion bar Q is pierced through the inner parts of these elements and attached by a key R. With this constitution, the object G is supported on the planetary gear B via a spring and simultaneosuly it is made to absorb or discharge the energy at the time of acceleration or deceleration of the weight.