To measure a wavelength characteristic of a gain of an optical amplifier highly accurately with ease and at a high speed upon the measurement for the same.

The input side of an optical amplifier 16 to be measured in provided with a multiple wavelength light source 10 for bringing the optical amplifier to a saturated state, and a wavelength select light source 12 for selectively outputting all measurement wavelength points other than the multiple wavelength light source by inputting a wide band light source 12a covering the whole measurement band into an optical filter 12b having a characteristic where it transmits light of the all measurement wavelength points other than the multiple wavelength light source but interrupts light other than the foregoing light. Further, the output side of the optical amplifier 16 is provided with an optical power meter 18, an optical spectrum analyzer 19, and a gain measurement section 20. Hereby, a gain characteristic with respect to wavelengths of the amplifier 16 can be measured highly accurately at a high speed continuously with ease.