1. An image processing apparatus, comprising: an image reading unit reading a front side image and a rear side image from a front side and a rear side of a medium, respectively; a mark detecting unit detecting a combination instruction mark present in a predetermined position of at least one of the front side image and the rear side image; and an image combining unit combining a front side sliced image and a rear side sliced image sliced from predetermined positions of the front side image and the rear side image in a predetermined direction to obtain one image when the combination instruction mark is detected.