A rotary encoder includes: a rotary disk with an angle code; a light source; a detector reading the angle code; and a processing unit acquiring a reading value. The light source includes at least two light-emitting elements spaced from each other. Every time the rotary disk is rotated by a predetermined angle, where an arbitrary angle from a rotation angle θ within a reading range on the detector is provided as φ, the processing unit acquires reading values fI(θ+φ) and fI(θ) with a first light-emitting element and a reading value fII(θ+φ) with a second light-emitting element, to calculate a reading value error due to deflection at an angle θ+φ based on the difference between the reading values fII(θ+φ) and fI(θ+φ), to obtain a difference gI(θ,φ) between the reading values fI(θ+φ) and fI(θ) such that the error is reflected, and to self-calibrate based on a change in the difference gI(θ,φ).