Negative absorption of an electromagnetic wave (2.6 GHz) in argon afterglows has been detected over the time interval between 2 and 10 ms at the pressures ranging from 0.5 to 0.9 Torr. Since the frequency 2.6 GHz is much greater than the collision frequency of electrons-atoms, the radiation could be collisional bremsstrahlung. We have followed the proposed model (Bekefi G et al 1961 Phys. Fluids 4 173-6) of the negative absorption which depends on the conditions of ∂f_e(v)/∂v>0 and ∂(v⁴Q(v))/∂v<0, and experimentally checked the existence of the these two requirements, where f_e(v) is the velocity distribution function of electrons, Q(v) a collision cross section for electrons. The former condition requires more electrons populating over higher-energy region on the electron velocity distribution function (EVDF). This may be overcome by assuming the generation of a high-energy electron hump created by ionizing collision between two metastable atoms. Experimentally, the occurrence of the high-energy electron group has been verified by the EVDF measured with the Druyvesteyn probe technique. The latter condition is satisfied by using argon gas which has a typical Ramsauer effect. We have come to a result that the calculated absorption coefficient by substituting the measured EVDF and the collision cross section into the theoretically derived formula (Bekefi G 1966 Radiation Processes in Plasmas (New York: Wiley)) agrees well with the measured value.