1. An image scanner comprising: a base having a front wall and a rear wall; a plate disposed on the base for the placement of a document which has a first surface to be scanned and is placed on the plate in a manner that the first surface faces upwardly relative to the plate; a transmitting stand disposed within the base and being capable of laterally moving therein; a top shell located above both of the plate and the base and extending to the front and rear walls of the base, the top shell being drivingly coupled with the transmitting stand to be laterally movable; and an optical sensing unit disposed in the top shell and comprising a lighting unit and a receiving unit, the lighting unit having a light emitting side and the receiving unit having a light receiving side with both the light emitting and receiving sides facing the first surface of the document to directly scan the document placed on the plate.