To provide a fiber increased in polarization maintaining ability while reducing the amount of polarization mode coupling and polarization mode dispersion, and to provide a highly efficient fiber amplifier or laser with a large outer diameter for maintaining polarization so that pump light is guided in an inside clad while a large outside clad ensures the decrease of fiber core inner mode coupling.

The polarization maintaining fiber has a fiber core of multimode having an elliptic cross section; a first clad that surrounds the fiber core; a second clad that surrounds the first clad, and a coating that surrounds the second clad. The first clad has a circular cross section, and the second clad has a noncircular cross section. Single mode light is propagated to the fiber core.