Cancellation of waves generated by four-wave mixing (FWM) in a single-mode fiber by use of midway optical phase conjugation is proposed and analyzed, and the possible power reduction is estimated. Effective cancellation of FWM is demonstrated with phase-conjugate waves generated by forward FWM in a zero-dispersion single-mode fiber, and a power reduction ratio of -13.9 dB is achieved in experimental three-channel transmission along a 40-km dispersion-shifted fiber.