At the S-300 generator (2 MA, 400 kV, 100 ns), the appearance of superhard X-ray radiation was observed during magnetic compression of cylindrical nested aluminum arrays with a linear mass of ~350 μg/cm, consisting of aluminum wires with a diameter of 15 μm. At the final compression phase of the arrays, a compact pinch is formed, consisting of a large number of hot spots located along the axis. This phase is accompanied by the emission of soft X-rays with a duration of ~10 ns. Simultaneously with the pulses of soft X-ray radiation, superhard X-ray radiation with an energy exceeding 450 keV was discovered. Superhard X-ray radiation was measured with shielded scintillation detectors with lead filters 20–70 mm thick. The main cause of overvoltage on the plasma column appears to be constriction instability.