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Decaying plasma in a microwave cavity filled with liquid helium (LHe, hereafter) is studied. An X-band mode cylindrical cavity is filled with LHe, and a high-voltage pulse with duration 7 s, voltage , and current is applied between a tungsten needle electrode and a thin stainless-steel mesh which separates an adjacent small discharge space from the cavity. The transmission signals through the cavity show the presence of decaying plasma in LHe with decay time on the order of 200 ms for a temperature range from 4.2 to 2.3 K at saturating vapor pressure. The response signals are suppressed in the pressurized LHe, as well as in superfluid LHe below 2.17 K. Our estimated mass of the charged carriers just after the pulsed discharge, assuming a diffusion loss mechanism, is much less than the value known as an effective mass of heavy ions which were produced by a steady-state ion source in LHe measured by Poitrenaud and Williams (1972). Our experimental observation suggests that the heavy effective mass of ions in LHe might take time to develop to its full extent.