High-speed shock-wave propagation in a cylindrical tube at low initial gas pressure has been investigated. A shock-wave was excited by fast magnetic compression of a plasma in a conical θ-pinch-type electrodeless device. The Mach number varied from 10 to 100, and the initial pressure from 20 μm Hg to 250 μm Hg. At high initial pressures the shock-wave properties were identical to those of a 'snow-plough' model. At low pressures an anomalous wave attenuation was observed which could not be explained by the existing theory. The shock-front width is in good agreement with that predicted by the ion-ion collision theory.