An interferometer using coaxial CO2and He–Ne lasers has been developed for electron density measurements in the reversed field pinch plasma. The interferometer uses the phase modulating heterodyne detection of 60 MHz. The CO2laser interferometer is free from the mechanical vibrations due to the reduction of the vibrations by the air springs and the compensation of the vibrations by the He–Ne laser interferometer. The electron density measurement during a long‐time discharge of 50 ms without the disturbance by the vibrations is achievable. The resolution is less than 1/100 fringe of the CO2laser interferometer, therefore the electron line density as small as 1.1×1014electrons/cm2is detectable.