To provide a driving transmission device and an image forming device which do not need large torque at start-up or deceleration, holds stable rotation with simple constitution, lower costs, saved space and constant speed and does not cause rotation deviation against static load torque.

This device is provided with a flywheel 20 as an inertial body rotatably mounted on a driven shaft 19 for rotating a photosensitive drum 5 and a pressing mechanism constituted by a pressing member 21 for pressing this flywheel 20 onto the side of a driven gear and rotating it with a frictional force. At acceleration/deceleration, as the flywheel 20 slips on the driven gear 18, a large motor torque is not needed, and the size, energy and space of the motor 16 can be saved. In the steady state, as the flywheel 20 rotates integrally with the driven gear 18, the inertial moment around the driven shaft is increased, and fitting vibration is reduced due to inertia effect.