A light amplifying device, which employs a SOA, has been disclosed, wherein, the gain of the SOA is adjusted by the injection of light into the semiconductor optical amplifier. The signal light and the CW control light are combined, entered into the SOA, and the control light is removed from the light output from the SOA by the filter, the amplified signal light is divided by the divider, and the power of one of the divided lights is detected. The control unit changes the power of the control light in accordance with the detected power. The gain is adjusted by changing the power of the control light. In this structure, the density of the carrier in the active layer is reduced by increasing the power of the control light in order to decrease the gain, but the saturation light output power of the SOA is increased because the carrier life is reduced.