A tangential chord CO2 laser interferometer for electron density measurement in the JT‐60U tokamak is described. To compensate for a phase shift due to mirror vibrations, an IR‐HeNe laser interferometer is introduced. Probe beams of the CO2 and the IR‐HeNe lasers travel in a similar form for a distance of 100 m by utilizing relay optics. The effective phase resolution of the system is estimated to be 1/68 fringe, which corresponds to a line integrated density of 3.1×1018 m-2. An ultra‐accurate phase comparator which has a 1/104 fringe resolution is proposed to improve the capability of present and future interferometers.