An apparatus includes a CCD that reads a script image, an analog front end unit (AFE) that adjusts the offset and the gain of an image signal and A/D-converts the image signal, an image processing IC that applies image processing such as shading correction, input gamma correction, color correction, MTF correction, output gamma correction to the digital image signal, and a spread spectrum frequency IC that generates a spread spectrum clock based on a source oscillation clock, wherein the former block of the image processing unit is driven with the source oscillation clock and the latter block is driven with the spread spectrum clock. The image signal is transferred asynchronously from the former block to the latter block through an external memory controller.