A simple structure of a digital photosensitive drum having an exposure source and a photosensitive member which are integrated with each other. The drum is mountable to a structure of a conventional electrophotographic image forming process. An interval between phase detecting patterns of an encoder wheel portion which is rotated with the drum is equal to or smaller than an interval between a charging position and a developing position. During an image forming process, a timing for each pixel to be driven to emit light is controlled based on a phase detection value.