An encoder able to maintain liquid-tightness over a long period of time. The encoder is provided with a housing unit holding component parts of the encoder. The housing unit is formed by a housing and a cover closing an opening part of the housing. At a boundary part of the housing and the cover, an O-ring is placed. The housing is formed with a guide groove at the outside in the radial direction from the O-ring and a discharge guide groove communicated with the guide groove and opening toward the outside in the radial direction. The cover is formed with a ring-shaped guide projection projecting out toward the guide groove at a position corresponding to the guide groove.