Abstract: A double cutting spindle unit comprising a main spindle unit and a sub-spindle unit mounted on the main spindle unit and vertically slidable relative to the main spindle unit. The chucking member is normally biased toward its fastening direction by a fastening spring formed by a plurality of spiral belleville springs arranged around the chuck actuating shaft. A two-stage hydraulic cylinder comprising two coaxially arranged cylinders is mounted on a supporting frame arranged on the top cover, and the actuating rod of the hydraulic cylinder is arranged with its free end being opposite to the end of the chuck actuating shaft.