Discharge time lag is experimentally studied when a single stepped voltage is applied to a gap with a stationary electron‐beam injection in a low‐pressure gas (0.06–2 mTorr). The time lag is found to be almost independent of the electron‐beam density. The beam density does, however, determine a discharge current, which is independent of the applied gap voltage and beam energy. A systematic measurement of the time lag as functions of the gap voltage, beam energy, gas pressure, and gap length is presented. The time becomes extremely short when the beam energy exceeds the ionization energy of the gas. The time lag, when a single pulsede‐beam is injected into the dc gap field, is also studied. The result is essentially the same in spite of pulsed operation.