PURPOSE: To perform conversion of an angular velocity by a method wherein one of the two focuses of an elliptical forms a rotary angle and teeth shapes are radially formed on the periphery of the elliptical centering around a rotary shaft.

CONSTITUTION: One focus of an elliptical 1 forms a rotary shaft 2, and teeh shapes are radially formed in the periphery of an elliptical centering around the rotary shaft 2. Gears in the same oval shape are engaged with each other in a position relation shown in a figure. When a drive shaft 3 is then rotated in the direction of an arrow mark 4, a driven shaft 5 is rotated in the direction of an arrow mark 6. At a current time, in the oval gear, when the driven shaft 3 is rotated in an angle A arc, the driven shaft 5 is rotated in an angle B arc. When the drive shaft 3 makes one full turn, the driven shaft 5 also makes one full turn. Similarly, since conversion of an angle is practicable, the efficiency of a conventional machine tool and engine can be improved, and the design of an energy saving and high efficient machine can take place.