NOVELTY - The device has an accessory support (2) with two support surfaces (21, 22). A variable position fixation unit (1) has a nut (11) with an inclined plane forming surface (14) to be in support on the accessory support, and a screw (12). The nut and the screw exert a pressure on an accessory and the accessory support, so that any pressure variation is transformed by the support surface (22) in an inclined plane into a force of translation of the accessory on the support directed against a force exerted on the accessory by an elastic belt.    USE - Used for adjusting position of an accessory such as a generator, a pump, a pulley e.g. crankshaft pulley, a ventilator and an alternator, mounted on a structure (claimed) such as a casing of a mechanical apparatus, and an engine such as an internal combustion engine.    ADVANTAGE - The nut has the surface to be in support on the accessory support, thus assuring an easy adjustment of the position of the accessory related to the structure and continuous adjustment of the tension of the elastic belt. The device uses the fixation unit at the same time for the displacement of the accessory related to the structure in order to tighten the belt, using the elasticity of the belt. The device permits the accessory to arrive in a position related to the structure in a precise manner with respect to the unit via which the belt having an optimal tension passes. The position of the accessory determined during the adjustment of the tension of the belt is not falsified by an immobilization tightening of the fixation unit, because the tightening between the screw and the nut has already influenced the tension of the belt in a direct manner. The accessory hence remains in the position reached at the present time, when an assembler stops acting on the fixation unit. The fixation unit does not damage the belt during the mounting, since the belt can be fitted with its length with hole, at the beginning of the mounting operation. The belt can be easily mounted on the pulleys and can be held as per the belt type and the mounting, according to the length of free sections between two successive pulleys, without risk of being damaged.    DESCRIPTION OF DRAWING(S) - The drawing shows a top view of an accessory position adjusting device.    Variable position fixation unit (1)    Accessory support (2)    Nut (11)    Screw (12)    Inclined plane forming surface (14)    Support surfaces (21, 22)