An error measurement device and an error measurement method are provided. The optical measurement assembly of the error measurement device includes a light source, an optical lens, and a photoelectric sensor. The light beam emitted by the light source is transmitted to a sensing area on the photoelectric sensor to form a first optical path illuminating on a first light-spot position of the sensing area. The moving stage is moved by a linear displacement, so that the light beam is transmitted to the photoelectric sensor to form a second optical path illuminating on a second light-spot position of the sensing area. The processor calculates a movement error of the moving stage and controls the actuator to drive one or more of the light source, the optical lens, and the photoelectric sensor to perform a relative motion, so that the light beam illuminates on the first light-spot position again.