PURPOSE: To form a semiconductor laser amplifier having an amplification factor independent of the wavelength of input light, by arranging a semiconductor laser, which an incident light of specified wavelength enters, on an optical path having a specified angle of reflection of the incident light of a specified wavelength.

CONSTITUTION: A diffraction grating 1 is used whose reflection angle changes in accordance with the difference of the wavelength of incident light. A semiconductor laser 3 which irradiates an incident light of specified wavelength is arranged on an optical path having a specified reflection angle of the incident light of specified wavelength. As a result, the reflection angle of the primary diffracted light reflected from the diffraction grating 1 changes in accordance with the difference of the wavelength of incident light, so that the angle of incident light into an active layer 6 of the semiconductor laser 3 is deflected, and the optical path in the active layer 6 changes in accordance with the difference of incident angle. Hence, when the semiconductor laser 3 is formed in the manner in which the optical path of the active layer 6 coincides with the length of a resonator, a semiconductor laser amplifier having an amplification factor independent of the wavelength of incident light can be formed. Thus it is realized that the gain of the semiconductor laser 3 does not change in accordance with the change of the frequency of signal light.