A scanner includes a first shaft, a second shaft, a photoelectric sensor for scanning a medium to produce an image, a movable carriage positioned on the first shaft for carrying the photoelectric sensor, a movable light source module positioned on the second shaft for generating a light beam for the medium, a driving module coupled to the carriage for driving the carriage to move a first offset along the first shaft, and a locking module coupled to the light source module and the carriage. As the carriage moves with the first offset, the carriage ensures the light source module moves a second offset along the second shaft through the locking module.