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A high power millimeter-wave beam generated by a 170 GHz gyrotron was injected into a cylindrical tube and atmospheric plasma was generated. The ionization front and a shock wave were propagated in the tube during the millimeter-wave pulse. From the pressure history measurement and plasma emission imaging in the tube, the propagation velocity of the shock wave was found identical to that of the ionization front. This indicates that isometric heating at the ionization front by millimeter-wave causes the efficient energy conversion from millimeter wave to fluid.