The optical fiber 1 for an optical fiber laser is provided with a rare earth element doped core 2 doped with a rare earth element, and a cladding 3 formed at an outer periphery of the rare earth element doped core 2. In the optical fiber 1 for an optical fiber laser, the rare earth element doped core 2 is divided into a plurality of core regions 2a, 2b, . . . , 2n−1, 2n along a longitudinal direction of the optical fiber 1 and dopant concentrations of the rare earth element in respective core regions 2a, 2b, . . . , 2n−1, 2n are different from each other, in order to flatten a temperature distribution of the optical fiber 1 along the longitudinal direction.