A rotating disc mower (10) with a lightened side mowing bar (12) comprising a fixed frame (13), a movable frame (14,14b) and a lightening device of the mowing bar (12), wherein the mowing bar (12) is positioned on a supporting arm (19) articulated with respect to the movable frame (14,14b), wherein a cylinder (28) is positioned between the supporting arm (19) and the movable frame (14,14b), which keeps the supporting arm (19) of the mowing bar (12) raised, as it is uploaded according to a pre-established force, a release rod (17) also being positioned between the fixed frame (13) and the mowing bar (12) and two cylinders (27) respectively positioned between the movable frame (14,14b) and the supporting arm (19), wherein a first hydraulic cylinder (25) is also provided, positioned between the fixed frame (13) and the release rod (17), and a second hydraulic cylinder (35), positioned between the movable frame (14,14b) and the supporting arm (19) of the mowing bar (12), wherein the first and second hydraulic cylinders (25,35) are hydraulically connected and activated by the release rod (17) which intervenes in the presence of a resistant force on the mowing bar, wherein said two cylinders (27) are constrained at a first end to a pin (31), passing into the supporting arm (19), and at the other opposite end to a pin (32) rotatingly arranged with respect to the movable frame (14) and positioned inside slots (33) of ears (34) that extend from the movable frame (14).
