A fiber laser oscillator (1; 100) has an optical fiber (10), a pumping light emitting device (30), a light guide member (40; 140), and a core member (50). The core member (50) is configured to have a flat plate-shaped, cylindrical, annular, or hollow spherical configuration. The optical fiber (10) is wound around the surface of the core member (50) or partially embedded within the surface of the core member (50). The light guide member (40; 140) has a light receiving surface (40a) and an opposite surface (40b) opposing the receiving surface. The opposite surface (40b) of the light guide member (40; 140) contacts the circumferential surface of the core member (50). Light (Lin) emitted from the light emitting device (30) travels (i.e., pumping light (Lin)) travels within the core member (50) along the circumference of the core member (50) and excites the core portion (12) of the optical fiber (10).