ROTATIONAL SPEED DETECTOR

PROBLEM TO BE SOLVED: To provide a rotational speed detector having small size and high output, capable of being housed in a limited space while a coil is hard to disconnect and excellent reliability is secured.SOLUTION: The rotational speed detector is constituted of a detection plate which is a rotating disk-like magnetic substance while recesses and protrusions are sequentially prepared in the circumferential direction, and an electric-magnetic conversion section for detecting the rotational speed of the detection plate. The electric-magnetic conversion section includes: a core which is a magnetic substance having a U-shaped cross section; a first permanent magnet prepared at one end of the core; a detection coil wound around one arm part of the core having the first permanent magnet at the end; and a second permanent magnet prepared at another arm part of the core not wound by the detection coil, or the second permanent magnet for forming the arm part, and the magnetic force of the second permanent magnet becomes stronger than that of the first permanent magnet.