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A late afterglow period of cryogenic plasma with density less than 108 cm-3 in 4.2-K helium gas is measured, where the plasma is lost mainly through ambipolar diffusion. We fabricate a large stainless-steel cylindrical discharge vessel of 16.6 cm in diameter and 8.2 cm in length. The vessel is a TE011 mode cavity with resonant frequency 2.85 GHz and Q-value larger than 3000. The diffusion length of the cavity, 2.1 cm, is much larger than those previously reported on cryogenic plasmas. A high-voltage pulse of 15 kV, 600 A with duration 2.5 μ s is applied between tungsten needle electrodes to produce a plasma repeatedly. Gas pressure is varied from 0.08 to 1.1 torr. Temporal changes in plasma density and electron-atom momentum transfer collision frequency are measured by an improved method of microwave interferometer including the cavity. The plasma decay with time constant on the order of 1 s is observed. We can see very slowly fading fluorescent light with our naked eyes. The decay time is increased, if gas temperature is cooled below 4.2 K.