An optical encoder includes a rotation disc, an annular slit, a light source, an absolute-value-related detecting unit, and a signal processing device. The annular slit includes a first annular slit and a second annular slit which are decentered in a mutually different direction. The light source is configured to irradiate light on the annular slit. The absolute-value-related detecting unit is configured to detect transmitted light or reflected light irradiated from the light source and transmitted or reflected at the annular slit to output a detected signal. The absolute-value-related detecting unit includes a first detecting unit corresponding to the first annular slit and a second detecting unit corresponding to the second annular slit. The signal processing device is configured to detect an absolute rotational angle of the rotation disc based on the detected signal output from the absolute-value-related detecting unit.