We estimated the plasma density produced by relativistic electron beam (REB) ionizing neutral gas, using both ionization theory and another method employed often to study energy losses of electron beam transporting through matter in nuclear physics. On experiment, Langmuir probe is employed to measure the plasma density produced by IREB ionizing N2, and the curve which plasma density varies with gas pressure was obtained. The experimental results indicate that plasma density in low pressure (1～15) region is in the order of , which coincides with theoretical estimation, and the method using probe to diagnose plasma produced by IREB ionizing neutral gas was feasible.