To meet the 5G mobile traffic demands, many small cells will be installed in the field. A promising candidate for reducing a large number of optical fibers connecting the central and distribution units is a tunable wavelength division multiplexing passive optical network. However, for systems in which multiple wavelengths are transmitted densely such as 100 GHz channel spacing, wavelength setting error and wavelength drift are major issues. In this paper, we describe a wavelength control method that uses an auxiliary management and control channel that complies with ITU-T G.989.3. Our method makes it possible to control the setting error of upstream signals at the initial connection between the optical line terminal and an optical network unit and also to control the wavelength drift due to the aging degradation of the laser diode. We also clarify the control conditions needed to minimize the control time.