An image forming apparatus is provided which is capable of forming images on recording materials of various types and sizes. The image forming apparatus includes a storing member for storing recording materials having an arbitrary size, a size-information input device for inputting size information corresponding to the arbitrary size of the recording materials, and a controller for controlling an image forming operation according to the size information inputted by the size-information input device. The size information input by the size-information input device includes each of arbitrary values corresponding to longitudinal and lateral lengths of the recording materials.