Optical transmission system transmits WDM signal from first node to second node via optical fiber. The optical transmission system includes: OCM that detects optical power of each wavelength channel in second node; processor that controls optical power of each wavelength channel based on detection by OCM in first node; optical circuit that adjusts optical power of each wavelength channel based on control signal from the processor in first node; and second processor that decides whether the optical powers of wavelength channels have converged to target level based on detection by OCM. When the optical powers of wavelength channels have not converged to the target level, the processor controls the optical circuit using the control signal in first cycle. When the optical powers of wavelength channels have converged to the target level, the processor controls the optical circuit using the control signal in second cycle longer than first cycle.