A frequency converter having no solid-state devices having nonlinear characteristics and no complex resonator structure and operable in a wide frequency range from microwave frequencies to terahertz wave frequencies. An input section (1) inputs an input wave into a high-frequency transmission line (2). A waveguide portion of an output section (6) reflects the input wave. A laser light source (4) inputs a laser beam into an optical delay circuit (5). The optical delay circuit (5) delays the laser beam and directs the delayed laser beam into a high-frequency transmission line (2) and a substrate (3). The laser beam transmitted through the optical delay circuit (5) produces an electron-hole plasma over the surface of the substrate (3) such as of a semiconductor thereby to short-circuit the high-frequency transmission line (2) and to reflect the input wave. The reflection point moves at high speed together with the laser beam, thereby converting the frequency on the same principle as the Doppler effect. The output section (6) total-reflects the input wave and takes out only an output wave.