A deceleration device comprising first and second gears that have different numbers of teeth. A cam mechanism is disposed on the same axis as the first gear, engages the first gear with the second gear in a state of eccentricity. A connecting shaft is disposed on the same axis as the first gear and is connected to the cam mechanism. A rotating body is disposed on the same axis as the second gear and is connected to the connecting shaft. When the connecting shaft is inserted into a long hole of the rotating body, the rotating body is connected to the connecting shaft in an eccentric position. The major axis of the long hole is set so as to allow the rotating body and the connecting shaft to move relative to one another along the longitudinal direction of the long hole.