[Problems to be Solved] To provide a rotary encoder capable of carrying out self-calibration for a horizontal encoder as well as a vertical encoder without requiring special equipment and simply and inexpensively with a high degree of accuracy while reducing the burden on a worker.[Means for Solving the Problems] In a rotary encoder including: a rotary disk (1) with an angle code (11); a light source that irradiates the angle code; a CCD linear sensor (3) that reads the angle code; and a processing unit that acquires a reading value f(θ) based on the angle code read by the CCD linear sensor, every time the rotary disk is rotated by a predetermined angle, where a rotation angle of the rotary disk is provided as θ and an angle from the rotation angle θ is provided as φ, based on a change in a difference g(θ,φ) between reading values f(θ+φ) and f(θ) within a reading range on the CCD linear sensor, the difference g(θ,φ) is determined as a Fourier series, and self-calibration of the reading value f(θ) of 0 to 360° is carried out based on the result.