Reflection resistances of respective fiber lasers in a state where an entire fiber laser system is in operation are evaluated. Each of fiber lasers (2 through 4) includes (i) a laser beam measuring section (28) configured to measure a power of a laser beam which a low reflection FBG (26) has transmitted therethrough and (ii) a Stokes beam measuring section (29) configured to measure a power of a Stokes beam which a high reflection FBG (24) has transmitted therethrough.