An optical device is equipped with an input/output module having at least one optical fiber, a movable mirror which deflects, toward the input/output module, light that is received from the input/output module, and a lens which couples the input/output module and the deflection unit to each other optically and has a focal length that is greater than or equal to 2.0 mm and shorter than 3.5 mm. A dispersion index ν of the optical device that is given by an equation: ν={n(1.45)−1}/{n(1.2)−n(1.7)} where n(1.45), n(1.2), and n(1.7) are refractive indices of a glass material of the lens at wavelengths 1.45 μm, 1.2 μm, and 1.7 μm, respectively, is larger than or equal to 100. The wavelength dependence of an optical characteristic of this optical device is weaker than that of conventional optical devices.