A dispersion interferometer based on CO2 laser for measurements of plasma line density in the gas dynamic trap (GDT) experiment has been developed with sensitivity <nel>min ~ 11013 cm-2, temporal resolution ~50 ns. The main advantages of the interferometer are compactness and low sensitivity to vibrations. The interferometer does not require specific vibration isolation structure and can be mounted directly on the working chamber of the plasma device. The above mentioned advantages have been successfully demonstrated in the Gas Dynamic Trap experiments.