In nonlinear optical crystal 183D, each step of wavelength conversion can be performed using another fundamental wave that is output from light output unit 161.sub.2, without using the fundamental wave whose output level has been attenuated after going through wavelength conversion for several steps (wavelength conversion in nonlinear optical crystals 183A to 183C). Therefore, the wavelength conversion efficiency improves and the peak power of optical amplifiers 161.sub.1 and 161.sub.2 can be suppressed, which allows the wavelength of lights that are output from optical amplifiers 161.sub.1 and 161.sub.2 to be further narrowed.