The fiber laser apparatus comprises an amplifying section for amplifying seed light by cladding-pumping, and has a structure for further using a residual component of pumping light for cladding-pumping in order to heat the object. A guiding optical fiber is provided between an amplifying optical fiber of the amplifying section and an output optical system converging the single-mode amplified seed light on the object, the guiding optical fiber serving to increase the degree of freedom in arranging the output optical system. The guiding optical fiber has a structure enabling the single-mode propagation of the amplified seed light outputted from the amplifying optical fiber and multimode propagation of the residual pumping light. Because the object is irradiated with the converged amplified seed light, while being heated with the residual pumping light outputted from the output optical system, even an object with a complex shape can be subjected to efficient laser processing.