The present invention relates to a wavelength-tunable light source whose output wavelength can be externally controlled and a wavelength-division multiplexed transmission system using the source.A wavelength-tunable light source in accordance with the present invention is constituted to be able to vary the output wavelength of a Fabry-Perot laser diode, that is wavelength-locked to an injected light, by controlling the wavelength of the injected light.A wavelength-tunable light source in accordance with the present invention provides comparatively large output power and excellent economic features.The present invention also presents a wavelength-division multiplexed transmission system using the wavelength-tunable light source.