Dispensing device for a viscous product with automatic volume adjustment comprising: - a cylinder with a piston and a piston rod, provided with a filling chamber with the product and a draining chamber of the filling chamber to apply the product; wherein the piston is displaced in the direction of the filling of the filling chamber due to the injection of the viscous product into said filling chamber, and is displaced in the opposite direction of the draining due the injection of the viscous product into the draining chamber, - a dosing pump to apply the product to, - a distributing base, provided with an hydraulic distributor of viscous product governed by an electrovalve, to distribute the product to a chamber selected between the filling chamber and the draining chamber; wherein the viscous product coming from the dosing pump, together with the viscous product coming out from the draining chamber, is applied during the filling of said filling chamber, - volume adjusting means of the filling chamber to adjust the amount of product to be dispensed; characterized in that it comprises: - an electronic controller with a display, configured to govern the amount of product to be applied and its application, - the volume adjusting means of the filling chamber comprise: ° an engine for the longitudinal displacement of ∘ a spindle, which position establishes the volume of the filling chamber, for which it constitutes a displacement stopper of the piston rod in the direction of the filling of the filling chamber, and for which the piston rod protrudes through the rear part of the draining chamber of the cylinder, ∘ an analog measuring instrument to measure the position of the spindle in order to position it in correspondence with the filling volume selected by means of the electronic controller, which receives a signal when said spindle is located in the position corresponding to the volume selected, - an inductive sensor, related to the spindle, for detecting when the piston rod enters into contact with the spindle and when the filling of the filling chamber takes place with the previously selected volume, which generates a signal indicating the filling that is detected by the electronic controller, - a mechanical sensor for detecting the end of the application when the filling chamber is drained and for detecting the start of the filling of said filling chamber, which generates signals indicating the end of the application and the start of the filling of the filling chamber, which are detected by the electronic controller. - a cylinder with a piston and a piston rod, provided with a filling chamber with the product and a draining chamber of the filling chamber to apply the product; wherein the piston is displaced in the direction of the filling of the filling chamber due to the injection of the viscous product into said filling chamber, and is displaced in the opposite direction of the draining due the injection of the viscous product into the draining chamber, - a dosing pump to apply the product to, - a distributing base, provided with an hydraulic distributor of viscous product governed by an electrovalve, to distribute the product to a chamber selected between the filling chamber and the draining chamber; wherein the viscous product coming from the dosing pump, together with the viscous product coming out from the draining chamber, is applied during the filling of said filling chamber, - volume adjusting means of the filling chamber to adjust the amount of product to be dispensed; - an electronic controller with a display, configured to govern the amount of product to be applied and its application, - the volume adjusting means of the filling chamber comprise: ° an engine for the longitudinal displacement of ∘ a spindle, which position establishes the volume of the filling chamber, for which it constitutes a displacement stopper of the piston rod in the direction of the filling of the filling chamber, and for which the piston rod protrudes through the rear part of the draining chamber of the cylinder, ∘ an analog measuring instrument to measure the position of the spindle in order to position it in correspondence with the filling volume selected by means of the electronic controller, which receives a signal when said spindle is located in the position corresponding to the volume selected, ° an engine for the longitudinal displacement of ∘ a spindle, which position establishes the volume of the filling chamber, for which it constitutes a displacement stopper of the piston rod in the direction of the filling of the filling chamber, and for which the piston rod protrudes through the rear part of the draining chamber of the cylinder, ∘ an analog measuring instrument to measure the position of the spindle in order to position it in correspondence with the filling volume selected by means of the electronic controller, which receives a signal when said spindle is located in the position corresponding to the volume selected, - an inductive sensor, related to the spindle, for detecting when the piston rod enters into contact with the spindle and when the filling of the filling chamber takes place with the previously selected volume, which generates a signal indicating the filling that is detected by the electronic controller, - a mechanical sensor for detecting the end of the application when the filling chamber is drained and for detecting the start of the filling of said filling chamber, which generates signals indicating the end of the application and the start of the filling of the filling chamber, which are detected by the electronic controller.