A wavelength-tunable light source includes a quantum cascade laser that emits light from a first end and a second end, an optical system that collimates the light emitted from the first end, a first reflecting section on which the light collimated by the optical system is made incident, and a second reflecting section that partially reflects the light emitted from the second end of the quantum cascade laser and transmits the remaining light. The first reflecting section includes a plurality of diffractive gratings whose diffractive properties are different from each other and whose lattice plane directions are variable, and the first reflecting section diffracts a light at a particular wavelength corresponding to the diffractive property and the lattice plane direction of the selected diffractive grating in the direction opposite to the incident direction.