SPEED DETECTION APPARATUS

PURPOSE:To attain to enhance detection accuracy, by providing a first magnetic circuit formed of a predetermined magnet to a detection apparatus and forming a second magnetic circuit passing through the detection apparatus and a moving body when the moving body approaches the first magnetic circuit. CONSTITUTION:When a shaft 1 is revolved and a moving body 4 is opposed to magnetic electrodes 22a, 23a, a second magnetic circuit phi2 connecting the moving body 4 and a magnet 12 is formed through both gaps 21a, 21b. Therefore, by the moving body 4, which is opposed to the magnetic electrodes 22a, 23a by the revolution of the shaft 1, and a slit, the intensity of the magnetic field passing through a magnetic optical element 24 comes to magnetic flux passing through a first magnetic circuit phi1 when the moving body 4 is not detected. When the moving body 4 is detected, the first magnetic circuit phi1 is distributed to the second magnetic circuit phi2 in parallel and, therefore, the intensity of the magnetic field passing through a magnetic optical element 24 becomes weak. The change in light from the magnetic optical element 24 by an optical fiber cable 5a is taken out on the basis of the change in the intensity of said magnetic field by an optical receiver.