The detecting device comprises a rotation member with a magnet portion which generates a magnetic field having different polarities arranged alternately in a circumferential direction, a case having a bearing, an angle calculating device and an elastic member which biases the rotation member so that the rotation member is rotated, keeping a constant inclined angle relative to a central axis line of the bearing or a phase difference between a rotation phase of the rotation member and a rotation phase of the bearing rotating about the central axis line thereof with an inclined angle relative to the rotation member is kept constant regardless of the change of the rotation angle of the rotation member.