A fiber fuse detecting apparatus provides early detection of a fiber fuse generation in an optical fiber that propagates laser light, preventing the damage from expanding. It comprises light input means arranged on a laser light incidence side of the optical fiber that inputs at least a portion of reverse direction propagation light propagating in a reverse direction to light incidenting the optical fiber, light receiving means that photo-electrically converts light input by the light input means, electric filter means that extracts from an output signal of the light receiving means a prescribed frequency band component that contains a periodic signal due to fiber fuse, detecting means that detects change of or a prescribed value in output level of the electric filter means, and warning means that emits a warning signal in response to a detection output of the detecting means. The laser light to the optical fiber is stopped or reduced by means of the warning means.