The authors have developed an angular position sensor which outputs an analog voltage in accordance with the angular position of rotation. The sensor detects the absolute angular positions of rotation and offers high precision along with high durability. The sensor detects the angular positions by means of optical detection of the relative distance between a spiral slit and a circular slit, which both have a coaxial center and are transparent. The nonlinearity of this sensor is less than ±0.5% and the detection range of the angular position is more than 355°. The operating temperature range is -30°C to +80 °C, with a thermal drift within 1° at the center of the detection range.