PROBLEM TO BE SOLVED: To provide a fiber which is decreased in the amount of polarization mode coupling and polarization mode dispersion and is increased in polarization maintaining ability and to provide a polarization maintaining high-efficiency fiber amplifier or laser of such a large external diameter that pumping light is guided within an inner clad while a larger outer clad ensures the decrease of the mode coupling in the fiber core. SOLUTION: The high-power polarization maintaining fiber taking the degree of the double refraction of the core, the sectional shape of the clad and the shape for polarization maintaining into consideration is decreased in the stress in the fiber core by increasing the fiber diameter to minimize the polarization mode coupling, by which the polarization mode coupling is minimized and the insensibility of the polarization state with respect to bending and temperature changing of the fiber on the outer side is achieved or the polarization mode coupling is minimized by optimization of the fiber coating in order to minimize the stress at the boundary between the fiber and the coating and eventually the insensibility to the polarization mode coupling is obtained by the value of the double refractivity drastically decreased as compared with the fiber of a small diameter.