In a revolution detecting device, a tunneling
magnetoresistance sensor having an element located in a region is provided. The tunneling
magnetoresistance sensor comprises a substrate, a pinned layer composed of
ferromagnetism material and located to one side of the substrate, a tunneling layer composed of insulating film and located to one side of the pinned layer and a free layer composed of
ferromagnetism film and located to one side of the tunneling layer. The element is configured to detect a change of
magnetoresistance of the element according to a
magnetic field applied in the region in which the element is located. The change of the magnetoresistance of the element is based on a change of current flowing through the tunneling layer between the pinned layer and the free layer. In the revolution detecting device, a revolution member is disposed in a vicinity of the element in the Y axis from a viewpoint of the element. The revolution member has a surface portion opposite to the element. The surface portion is formed with S poles and N poles which are alternately arranged. In the revolution detecting device, a
magnet is disposed in a vicinity of the element and generating the
magnetic field and a direction of the
magnetic field is substantially parallel to the Y axis at a center portion of the element. When the revolution member revolves, the S poles and N poles are configured to move substantially in parallel to the X axis on the Y axis determined by the element.