To provide an optical amplifier for amplifying multiple wavelengths that monitors an input output signal light for each wavelength.

An optical branch section 20 branches an input light to an optical amplifier section 30 to extract an optical signal with a wavelength, in response to temperature and light receiving units 25-1-25-n receive lights of each wavelength. An input level monitor circuit 26 generates a signal reflecting a power level of the input light to the optical amplifier section 30 from the signal received from the light receiving units and an A/D converter circuit 27 applies A/D conversion to the signal. Similarly, a signal reflecting a power level of an output light of the optical amplifier section 30 is obtained by a circuit block, consisting of an optical branching section 40 to an A/D conversion circuit 47. Temperature control circuits 24, 44 change each element temperature of arrayed waveguide gratings(AWG) 21, 41 to control the wavelength of a transmission light of each AWG to acquire the power levels of a signal component and a noise component. A processing circuit 60 calculates the gain and noise figure of the optical amplifier section 30 from each acquired power level.