PURPOSE: To provide a driving device capable of surely and efficiently transmitting the rotational force of a driving source to an object to be driven by using a belt with a proper tension irrespective of a driving state in the case of using the driving source such as an electric motor, etc., while switching the continuous drive and an intermittent drive.

CONSTITUTION: When the electric motor 4 is continuously driven, a lever 100 is rotated around a shaft 100a clockwise by an adjusting motor 101, a pressing force against a toothed belt 6 by a tension roller 12 is reduced so as to reduce the tension of the belt. When the electric motor 4 is intermittently driven by a pulse voltage, the lever 100 is rotated around the shaft 100a counterclockwise by the adjusting motor 101, and the pressing force against the toothed belt 6 by the tension roller 12 is increased so as to increase the tension of the belt.