The polarization-maintaining optical fiber 10 of the present invention is designed so that a plurality of core portions 12a,12b which have a high refractive index is provided in parallel along a single diameter direction in the cross section of the optical fiber, these core portions 12a,12b cooperating to propagate a single fundamental mode. In the production method for the polarization-maintaining optical fiber of the present invention, a plurality of holes 22 are formed longitudinally in parallel along a single diameter direction of a glass rod 21 having a low refractive index which forms the cladding, glass rods 23 for core use having a high refractive index which form the core portions are inserted into these holes 22, heating to form a unitary body is carried out, creating a preform which is then drawn. Further, the rare-earth-doped polarization-maintaining optical fiber of the present invention uses a rare earth element to dope the optical waveguide portion of the polarization-maintaining optical fiber, and can be employed in a light amplifier or laser oscillator. Moreover, the polarization-maintaining optical fiber coupler of the present invention is formed by bringing two or more polarization maintaining optical fibers into contact, heating, fusing and elongating them, and heating the vicinity of the connection point before and after connection.