A 2-mm wave digital interferometer without source frequency modulation was fabricated to measure the central electron density of tokamak discharges in an upgraded DIVA device. Two methods of processing the interferometic fringe signals were presented: one by using an up/down counter and the other by a computer to obtain direct graphic displays of the complicated time-evolutions of the average electron density. These methods were successfully put into operation with sufficient resolutions of a 1/2 fringe by the counter and a 1/40 fringe by the computer, and the utility of the digital interferometer was fairly evident in practical measurements. The central average density was found to reach 4.2×1013 cm-3 in the upgraded DIVA plasmas by using the present digital interferometer.