The invention relates a protective structure for brushes of sweeping machines (30) having a frame (31) and at least one cup-shaped brush (20) supported protruding from the frame (31), said structure (1) comprising a flexible membrane (2), positioned near the cup-shaped brush (20) and suitable to prevent the diffusion of dust in the environment, and a supporting system (3) for the flexible membrane (2), the supporting system (3) being movably connected to the sweeping machine and comprising a movable portion (6) which shapes the flexible membrane (2) and elastically deforms near the frame (31) counter to elastic means (9).