Abstract: A doppler detector for measuring the speed and/or the direction of a moving body utilizing the principle of the doppler effect that the reflected frequency from a moving body is slightly changed has been found. The present doppler detector comprises a rectangular waveguide inserted between a microwave oscillator and an antenna, an elongated ferri-magnetic body inserted between a pair of H planes of said waveguide, and said body being magnetized in the axial direction of the same, and a pair of diodes each inserted between H planes of said waveguide so that the first diode being symmetrical with the second diode concerning the lateral plane including the center axis of said ferri-magnetic body.