A fiber laser apparatus includes an optical amplification fiber with a multi-core structure composed of a first waveguide that pumping light enters and that transmits the pumping light; a second waveguide composed of a core containing a laser medium and for generating laser, and a clad for transmitting pumping light; and a third waveguide containing the first waveguide and the second waveguide. The optical amplification fiber is wound while the curvature radius is being changed to provide a small-size, high-power fiber laser apparatus.