A rotary disc mower with a side mowing bar comprising: - a fixed frame, - a movable frame and - a lightening device of the mowing bar, wherein said mowing bar is positioned on a supporting arm articulated in a pin with respect to the movable frame, wherein a lightening cylinder is positioned between said supporting arm and said movable frame, which keeps said supporting arm of said mowing bar lightened, said cylinder being loaded according to a predetermined force, a release rod also being positioned between said fixed frame and said movable frame carrying said mowing bar, and respectively a lifting cylinder between said fixed frame and said supporting arm, wherein a first hydraulic cylinder is also provided, positioned between said fixed frame and said release rod, and a second hydraulic cylinder, positioned between said movable frame and said supporting arm of said mowing bar, wherein said first and second hydraulic cylinders (25,35) are hydraulically connected and actuated by said release rod which intervenes in the presence of a resistant force on the mowing bar, wherein said lifting cylinder is constrained at a first end to a pin, passing through the supporting arm, and at the other opposite end to a pin rotatingly positioned with respect to said fixed frame, characterized in that at least one fork rotatable around said pin is also provided, which engages said pin of said lifting cylinder connected via a pin to a stem of said second hydraulic cylinder, positioned between said movable frame and said supporting arm. - a fixed frame, - a movable frame and - a lightening device of the mowing bar, wherein said mowing bar is positioned on a supporting arm articulated in a pin with respect to the movable frame, wherein a lightening cylinder is positioned between said supporting arm and said movable frame, which keeps said supporting arm of said mowing bar lightened, said cylinder being loaded according to a predetermined force, a release rod also being positioned between said fixed frame and said movable frame carrying said mowing bar, and respectively a lifting cylinder between said fixed frame and said supporting arm, wherein a first hydraulic cylinder is also provided, positioned between said fixed frame and said release rod, and a second hydraulic cylinder, positioned between said movable frame and said supporting arm of said mowing bar, wherein said first and second hydraulic cylinders (25,35) are hydraulically connected and actuated by said release rod which intervenes in the presence of a resistant force on the mowing bar, wherein said lifting cylinder is constrained at a first end to a pin, passing through the supporting arm, and at the other opposite end to a pin rotatingly positioned with respect to said fixed frame, characterized in that at least one fork rotatable around said pin is also provided, which engages said pin of said lifting cylinder connected via a pin to a stem of said second hydraulic cylinder, positioned between said movable frame and said supporting arm. wherein said mowing bar is positioned on a supporting arm articulated in a pin with respect to the movable frame, wherein a lightening cylinder is positioned between said supporting arm and said movable frame, which keeps said supporting arm of said mowing bar lightened, said cylinder being loaded according to a predetermined force, a release rod also being positioned between said fixed frame and said movable frame carrying said mowing bar, and respectively a lifting cylinder between said fixed frame and said supporting arm, wherein a first hydraulic cylinder is also provided, positioned between said fixed frame and said release rod, and a second hydraulic cylinder, positioned between said movable frame and said supporting arm of said mowing bar, wherein said first and second hydraulic cylinders (25,35) are hydraulically connected and actuated by said release rod which intervenes in the presence of a resistant force on the mowing bar, wherein said lifting cylinder is constrained at a first end to a pin, passing through the supporting arm, and at the other opposite end to a pin rotatingly positioned with respect to said fixed frame, characterized in that at least one fork rotatable around said pin is also provided, which engages said pin of said lifting cylinder connected via a pin to a stem of said second hydraulic cylinder, positioned between said movable frame and said supporting arm.