An image sensor unit includes a reflection reading light source that illuminates a bill, a substrate on which the reflection reading light source is mounted, a reflection reading light guide that guides the light emitted by the reflection reading light source to the bill, a rod-lens array that focuses the light from the bill, a light receiving element that receives the light from the rod-lens array, a sensor substrate on which the light receiving element is mounted, and a frame that houses the substrate, the reflection reading light guide, the rod-lens array and the sensor substrate. The sensor substrate has a passage hole and is disposed closer to the bill than the substrate. The frame is disposed with the reflection reading light guide inserted into the passage hole, so that the light from the reflection reading light source reaches the bill through the passage hole.