A wavelength-variable light source according to the present invention includes 2×2 3-dB directional coupler 3, closed loop-type optical circuit 5, at least two resonators 1 and 2, and optical amplifier 4. The closed loop-type optical circuit 5 is formed by connecting ends of the two output paths of 3-dB directional coupler 3. The resonators 1 and 2 have different resonance wavelength periods. One end of optical amplifier 4 is optically connected to one input path end 6 of 3-dB directional coupler 3. Lasing light is output from the other end of the optical amplifier 4. A non-reflecting structure is formed at the other input path end 7 of the 3-dB directional coupler. The wavelength-variable light source configured as described above includes an element configured to vary the resonance wavelength of the resonator 1 or 2.