An encoder for detecting rotation information of a first rotation shaft includes a first detector which detects a pattern provided in the first rotation shaft, a first housing case which houses the pattern provided in the first rotation shaft and the first detector, a second housing case which houses at least a part of a shaft portion different from a portion where the pattern is provided, of the first rotation shaft, and an adjusting mechanism which adjusts at least one of an atmospheric pressure and a temperature in the first housing case to be higher than at least one of an atmospheric pressure and a temperature in the second housing case. The inflow of the high-humidity air from a motor into the encoder is prevented or suppressed, so that the condensation in the encoder can be prevented or suppressed.