A control method, which is applicable to a variety of network configurations, controls an optical transmission system to determine optimum optical input power to a transmission path for increased optical transmission quality. The optical transmission system has terminal stations, repeaters, dispersion compensation modules, and a dispersion compensation controller. The terminal stations transmit and receive an optical signal through an optical fiber transmission path. The repeaters are disposed in the optical fiber transmission path for amplifying the optical signal. The dispersion compensation modules are disposed in the terminal stations and the repeaters for compensating for dispersion of the optical signal. The dispersion compensation controller determines a target value for a nonlinear phase shift, which is an indication of a self-phase modulation caused by a transmission medium of the optical fiber transmission path, to have a maximum residual dispersion tolerance, and adjusts at least one of the input powers applied to transmission paths connected to the terminal stations or the repeaters and the input powers applied to the dispersion compensation modules to equalize the nonlinear phase shifts of paths in the optical transmission system to the target value.