A photoelectric recording interferometer for gas analysis is reported. Central part of interference fringes is projected on the single slit of a photomultiplier through a polarizer P1with its principal axis perpendicular to fringes and a field splitting polarizer P2whose two Polaroid planes are split by a line parallel to fringes and oriented to ±45° to the principal axis of the polarizer P1and a rotating Polaroid disk. When the center of a fringe is brought in line with the splitting line of polarizer P2, alternating photocurrent becomes zero. Any change of position of the center line of the fringe relative to the splitting line generates an alternating photocurrent with an amplitude which is a function of the fringe shift and a phase which is a function of the direction of the shift. The shift of fringes which is proportional to gas concentration is detected; this shift makes a servo‐motor start to put the fringes back to their original position optically. The angle of rotation of the servo‐motor is proportional to the shift of fringes indicating the variation of the refractive index, i.e., the variation of the content ratio of the mixed gas. The sensitivity of this apparatus is observed to be about 1×10−8index unit, or in the case of CO2gas in air, about 0.01% by volume.