A terminal apparatus by which, even where BER data, for example, of an optical signal of an existing wavelength cannot be utilized, the optical output powers of the existing wavelength and an extension wavelength can be adjusted in a short period of time while eliminating quality degradation of the existing circuit. The terminal apparatus includes a level controlling section for controlling a total optical output power of an existing wavelength signal light and an extension wavelength signal light to a fixed level, a monitoring unit capable of monitoring the optical output power for each wavelength, and a controller for controlling the optical output power of the extension wavelength signal light so that the optical output power of the existing wavelength signal light monitored by the monitoring unit may not be lower than a minimum permissible optical output power.