Dynamics of a fast-flowing plasma through a magnetic mirror field was investigated. A highly-ionized, high-density, He plasma produced by a quasi-steady MPD arcjet (MPDA) was injected into a magnetic mirror. In a uniform magnetic field region, ion acoustic Mach number (Mi) was almost unity, while in a diverging field region the Mach number increased up to 2-3. When the supersonic plasma flows into a converging field region, a shock-like structure was formed. The subsonic flow downstream of the shock was re-accelerated up to Mi of 2-3. The sonic condition (Mi=1) is satisfied at the magnetic mirror throat as in a conventional Laval nozzle. The adiabatic exponent of ions was evaluated by comparing measured spatial profiles with the prediction from 1D isentropic model.