A hydrogen cyanide laser interferometer with three vertical chords was installed in the JFT‐2M tokamak for electron density measurement. The optical arrangment of the interferometer is a Mach–Zehnder type and its structure is mechanically independent of the JFT‐2M machine in order to suppress the mechanical vibration. Two HCN lasers are employed for direct reading of the chord averaged electron density by means of beat modulation method. The beat frequency between two HCN lasers is stabilized at a constant frequency by a feedback circuit using a microcomputer. A combination of short dielectric waveguides and TPX lenses is employed to compensate the refraction and divergence of the laser beam. It was shown experimentally that the interferometric measurement of the electron density was scarcely affected by the mechanical vibration of the interferometer and the refraction of the probing laser beam up ton¯e=6×1019m−3.