To provide a rotation angle detecting device, in which undesirable noises from an angle calculating circuit can be reduced to increase a detection accuracy, outputs of four linear magnetic sensor arrays arranged on a non-rotatable member confronting a magnetic generating element and arranged in a plane perpendicular to the axis of rotation, the sensor arrays being arranged along four sides of an imaginary rectangular shape, are read out by signal read-out circuits, and then converted into digital signals by AD converting circuits in order to calculate an angle of rotation of the magnetic generating element by an angle calculating circuit. The signal read-out circuits and the AD converter circuits are arranged outside of the magnetic sensor arrays arranged in a generally rectangular pattern, and the angle calculating circuit is arranged inside of the magnetic sensor arrays, all being mounted on a semiconductor chip.