To obtain a wavelength sweep light source which can sweep light of a narrow spectrum line width and can be formed at low cost.

The wavelength sweep light source has a semiconductor optical amplifier 10, a ringed light feedback system 11 which returns light emitted from one edge face 10a of the semiconductor optical amplifier 10 to the element 10 from the other edge face 10b and performs laser oscillation, a fiber Fabry-Perot etalon 12 which transmits light traveling inside the ringed light feedback system 11 through wavelength selection and continuously changes the selected wavelength, and a means 17 such as a coupler which takes out a part of the light traveling inside the ringed optical feedback system 11 to an outside. The semiconductor optical amplifier 10 emits light which is subjected to straight line polarization in a prescribed direction. The ringed light feedback system 11 is constituted of polarization maintaining fibers 20, 21. A pair of /4 boards 13, 14 disposed with the fiber Fabry-Perot etalon 12 therebetween and a pair of polarizers 15, 16 disposed outside the /4 boards 13, 14 are provided inside the ringed light feedback system 11.