Abstract: An MO-PA fiber laser having a master oscillator; and a first power amplifier which uses as a gain medium, a rare earth-doped optical fiber which is connected to a later stage of the master oscillator, wherein the MO-PA fiber laser has a wavelength conversion portion between the master oscillator and the power amplifier, and has a wavelength filter between the wavelength conversion portion and the master oscillator which only allows wavelength components of pulse light emitted from the master oscillator to pass, thereby making it possible to prevent breakage to a fiber laser which is caused by reflection light without using high-cost optical components.