In our work, we performed experiments to examine the conditions under which a runaway electron beam moves not only toward the anode but also backward (to the space downstream of the cathode). In the experiments, a SLEP-150 generator was used which allowed us to obtain 6·1010 electrons in atmospheric pressure air downstream of an Al foil anode 10 m thick [1]. The design of the SLEP-150 generator is described in [1, 2]. For recording a backward runaway electron beam, a positive voltage pulse was applied to the high-voltage electrode of the gas diode. The amplitude of the voltage pulse in the incident wave was about 150 kV and its FWHM was about 1 ns. The voltage pulse rise time was determined by a peaking switch and was 0.3 ns.