The present invention achieves a fiber laser having a high reflection resistance. A length of an optical fiber MMF is set so that a condition is satisfied at each point on an individual optical path of a fiber laser (FL2), the condition being that a difference between time at which a power of a forward Stokes beam (SF) is at a maximum value and time at which a power of a backward Stokes beam (SB) is at a maximum value is greater than a sum of a half width at half maximum of the power of the forward Stokes beam (SF) and a half width at half maximum of the power of the backward Stokes beam (SB).