Disclosed is an image acquiring method. The image acquiring method includes receiving an instruction to acquire a one-side image to acquire an image on a first side of a document; obtaining a penetrated image of the first side of the document, which is seen on the second side, with a second image sensor for obtaining an image on a second side of the document, while the document is being fed; obtaining an image on the first side of the document with a first image sensor for obtaining an image on the first side of the document, while the document is being fed; and processing the image on the first side based on the penetrated image of the first side.