Provided herein are a method and apparatus for controlling the wavelength of an external-cavity wavelength-tunable laser. The reflectivity or transmittance characteristics of a wavelength depend on the phase difference between light reflected by a wavelength-tunable filter and parasitic reflective light. Thus, when the temperature of a module and current applied to a phase control electrode and an external reflector are changed, the characteristics are changed in a periodic manner. The present disclosure relates to a wavelength control algorithm of determining combinations of these parameters to set conditions having optimum chirping characteristics at a desired wavelength.