A rotary encoder is for detecting the rotation position or rotation speed of a shaft. The rotary encoder includes: a rotation code plate having a detection code and to be rotated by the shaft; and a boss attached to the rotation center of the rotation code plate and used for attaching the rotation code plate to the shaft. The boss is made of a resin material. The boss includes a fitting hole formed at the rotation center of the boss and used for making a fit with the shaft through press-fitting.