In an optical transmission apparatus, optical power of each of wavelength contained in a wavelength multiplexing signal is measured, a measured value of the optical power of each wavelength is compared with a predetermined threshold value, a total value of the measured values of the optical power, which are equal to or larger than the threshold value, is calculated as a total value of signal beam power, and a total value of the measured values of the optical power, which are less then the threshold value, is calculated as a total value of ASE power. The number of the measured values of the optical power, which are equal to or larger than the threshold value, is calculated as a wavelength count of the signal beams contained in the wavelength multiplexing signal. A signal-to-ASE ratio is calculated from the total value of the signal beam power and the total value of the ASE power. The optical power of the wavelength multiplexing signal inputted to an optical amplifier is measured, and a target value of the automatic level control (ALC) is determined by use of the measured values of the wavelength count, the signal-to-ASE ratio and the optical power of the wavelength multiplexing signal in order to execute the ALC of the optical amplifier so that the optical power of each of the wavelengths in the wavelength multiplexing signal outputted from the optical amplifier, becomes constant.