There is described a signal processing apparatus, which conducts processing of a signal outputted from an image capturing element that converts an optical image obtained by irradiating light on an object such as document to an electric signal. The apparatus includes: a sample-and-hold section to sample and hold a black signal level and an image signal level, both included in an analogue image signal inputted from an image capturing element; an analogue-to-digital converting section to convert the analogue image signal to digital image data within a predetermined number of conversion steps, which is larger than a number of conversion steps corresponding to a differential value between the black signal level and the image signal level; and a digital subtracting section to subtract black digital-image data representing the black signal level from signal digital-image data representing the image signal level, both being converted by the analogue-to-digital converting section.