Abstract: Variations in a peak power level of add signals due to transmission trouble are to be reduced with a relatively inexpensive and simple configuration. When an input of optical signals to an optical amplifier unit is cut off by transmission trouble, only amplified spontaneous emission light is supplied from the optical amplifier unit. An output power constant control unit amplifies this amplified spontaneous emission light to a certain level. This amplification level is substantially equal to what it would be if the input optical signals were inputted into the optical amplifier unit. This makes it possible to restrain variations in a ratio of OADM add signals to be added to OADM through signals in an OADM unit in time of transmission trouble.