PURPOSE: To provide an optical amplifier in which deterioration of an S/N ratio due to natural radiation light noise is small and a decrease in a saturated level when connected in cascade is small.

CONSTITUTION: An optical signal is amplified by a rare earth element series- doped optical fiber 1 excited by an exciting light source 2, and a component perpendicular to a polarization plane of the signal of a natural radiation light included in an output light is removed by a polarization limiter 6. Thus, an optical amplifier having a small nose index and a small decrease in a saturated level when connected in cascade is constituted.