<p>There is provided a light wavelength branch insert device capable of performing optical output power constant control for flattening the optical signal level of each channel as a passing light with a simple structure not requiring a special trigger signal and with a high reliability from the system rise at a high speed. The optical output power constant control unit (12) controls variable light attenuation means so that a fixed attenuation amount is given to each light wavelength signal in the inputted wavelength multiplex light signal until the monitor result of the power monitor means exceeds the optical output power constant control threshold value. When the monitor result exceeds the threshold value, the optical output power constant control unit (12) starts the control operation of the variable light attenuation means so as to give a variable attenuation amount required for reaching the target value of the output optical power constant control for flattening the wavelength multiplex signal level transmitted to the downstream light transmission path, to each light wavelength signal in the inputted wavelength multiplex light signal.</p>
