A speed limiting device for a sliding door, the speed limiting device comprising: - a tightly closed casing (12, 14), internally provided with at least one hydraulic circuit for the circulation of a hydraulic fluid, said casing (12, 14) being arranged to be made integral with a panel of the sliding door, and said hydraulic circuit being provided with at least one flow control valve suitable for controlling the hydraulic fluid flow circulating internally to said hydraulic circuit; - a motion transmission means, located outside the casing (12, 14) and arranged to engage with a corresponding transmission element placed on the sliding door frame; and - a gear unit (20, 22, 24), located inside the casing (12, 14) and in fluid communication with the hydraulic circuit, said gear unit (20, 22, 24) being connected to the motion transmission means through at least one transmission shaft and being driven in rotation by said motion transmission means to move the hydraulic fluid inside the hydraulic circuit, the speed limiting device operating as a hydraulic gear pump in which, by generating the rotation of said transmission shaft and said gear unit (20, 22, 24), the flow of said hydraulic fluid is moved all along a known path through said hydraulic circuit, the speed limiting device being characterized in that said gear unit (20, 22, 24) consists of a speed increasing gearbox comprising four or more toothed wheels (20, 22, 24) with a different number of teeth, said speed increasing gearbox comprising in cascade, starting from the transmission shaft, a first toothed wheel with a first number of teeth, a second toothed wheel with a second number of teeth, a third toothed wheel with a third number of teeth and a fourth toothed wheel with a fourth number of teeth, wherein said fourth number of teeth is smaller than said first number of teeth. - a tightly closed casing (12, 14), internally provided with at least one hydraulic circuit for the circulation of a hydraulic fluid, said casing (12, 14) being arranged to be made integral with a panel of the sliding door, and said hydraulic circuit being provided with at least one flow control valve suitable for controlling the hydraulic fluid flow circulating internally to said hydraulic circuit; - a motion transmission means, located outside the casing (12, 14) and arranged to engage with a corresponding transmission element placed on the sliding door frame; and - a gear unit (20, 22, 24), located inside the casing (12, 14) and in fluid communication with the hydraulic circuit, said gear unit (20, 22, 24) being connected to the motion transmission means through at least one transmission shaft and being driven in rotation by said motion transmission means to move the hydraulic fluid inside the hydraulic circuit, the speed limiting device operating as a hydraulic gear pump in which, by generating the rotation of said transmission shaft and said gear unit (20, 22, 24), the flow of said hydraulic fluid is moved all along a known path through said hydraulic circuit, the speed limiting device being characterized in that said gear unit (20, 22, 24) consists of a speed increasing gearbox comprising four or more toothed wheels (20, 22, 24) with a different number of teeth, said speed increasing gearbox comprising in cascade, starting from the transmission shaft, a first toothed wheel with a first number of teeth, a second toothed wheel with a second number of teeth, a third toothed wheel with a third number of teeth and a fourth toothed wheel with a fourth number of teeth, wherein said fourth number of teeth is smaller than said first number of teeth.