The fiber laser beam processing apparatus includes a fiber laser oscillator, a laser power source unit, a laser injecting unit, a fiber transmission system, a laser beam irradiating unit, a processing table, etc. The fiber laser oscillator includes an optical fiber for oscillation, an electric optical pumping unit that applies an excitation beam for optical pumping onto an end face of the optical fiber for oscillation, and a pair of optical resonator mirrors optically facing each other through the optical fiber for oscillation. The optical fiber for oscillation includes a core extending on the central axis thereof, a clad surrounding the core, an air layer surrounding the clad, and a support surrounding and supporting the air layer.