To provide a velocity detector using an optical sensor which has a required accuracy even in a lower velocity region at a single step axle driving rotation ratio in stead of a conventional tachometer generator method.

Each of photodetector parts 8/9 which detects passing/reflective light of semiconductor laser-beam from a luminous part 7 by photodiodes or the like, is installed by using optical disk, for example CD(compact disk) or the like, which is rotated by an axle 2 and has circumferentially plural passing light detection small holes 6 with equal pitches for passing light of semiconductor laser-beam.