PURPOSE: To increase the value of permissible input power which is limited by four-light-wave mixing by cascading plural double refraction media so that their optical main axes mutually slant.

CONSTITUTION: Double refraction optical fibers 1 and 2 as the double refraction media have polarization states of signal light rotated by different angles for each light frequency. The optical fibers 1 and 2 are cascaded having their optical main axes slanted to each other. The inclination of the optical main axis between the optical fibers 1 and 2 is almost 45°. Consequently, even if signal light matching the main axis direction of the optical fiber of the front stage is inputted, its polarization state can be shifted in the rear stage. This four-light- wave mixed light suppressing circuit is decentralized and interposed in an optical fiber transmission line which transmits light frequency multiplexed signal light at substantially equal frequency intervals. Consequently, the worst state wherein all light signals are transmitted in the same polarization state throughout the transmission line is eliminated.