Belt stretcher for serial printer wherein a spring (14) imparts a predetermined pull to a driving belt (2) by applying a predetermined force to a slide (8) on which a belt return idle pulley (10) is mounted, and wherein slide offsets from a working point and in the direction opposite to the one of the force exerted by the spring (14) are minimized by a stop pad (18), preferably resilient and position adjustable, or automatically changeable in position owing to belt elongations occurring during operation.