In accordance with an optical modulator using waveguides of the present invention, the reflection phenomenon of light is used, so that the optical modulator is neither sensitive to the wavelength, mode, polarization, intensity, beam size, etc. of light, nor sensitive to the temperature, the waveguide size, the uniformity of a refractive index, carrier doping concentration, changes in control voltage/current, etc., thus obtaining stable optical modulation characteristics. An optical modulator using waveguides includes a first waveguide (11) configured to allow an optical signal to be incident thereon, and formed in a direction identical to that of the incident optical signal. A second waveguide (12) is formed to branch from the first waveguide in a first direction. A reflector (13) is arranged in a region in which the second waveguide branches from the first waveguide. A controller (14) is configured to control a refractive index of the reflector.