An imaging device includes a light source; a prism having an imaging surface to be contacted with an object; and an image-forming optical system that forms an image of the object. The image-forming optical system includes a front lens group having a first optical axis tilted relative to the imaging surface; an image plane tilted from a surface perpendicular to the first optical axis of the front lens group; and a rear lens group having a second optical axis shifted in parallel to the first optical axis of the front lens group. The image-forming optical system is arranged to satisfy the following conditional expression: |Aim|/(90°−Ain)