A rotation angle detecting device includes a columnar permanent magnet that has a generally ring-shaped cross-section and a pair of magnetic poles, a pair of semi-cylindrical yokes disposed in a circumferential direction opposite the permanent magnet, a rotary shaft fixed to the permanent magnet at a shifted rotation axis and a magnetic sensor disposed in one of circumferential gaps between the yokes. The rotation axis and the magnetic poles are arranged so that the magnetic flux density becomes maximum at portions of the yoke less than 90 degrees angled from the circumferential gap in which the magnetic sensor is disposed.