1. An exposure device for exposing a photosensitive medium to light on the basis of image data containing multiple pixels, comprising: a light source; a scanning mechanism for scanning on a photosensitive medium light emitted from the light source; deriving means for deriving a pixel level between the pixel levels of two neighboring pixels contained in the image data; first control means for controlling the light source and the scanning mechanism so as to form an exposed region on the photosensitive medium by exposing the photosensitive medium to the light emitted from the light source according to the pixel level of each pixel contained in the image data; and second control means for controlling the light source and the scanning mechanism so as to expose the photosensitive medium to the light emitted from the light source according to the pixel level derived by the deriving means in the region between the centers of two exposure regions corresponding to the two neighboring pixels responsible for the derivation of the pixel level by the deriving means.