An image forming system has on original size detector, a paper size selection key, a magnification factor selection key, a paper size automatic selection key, a magnification factor automatic set key, a display for displaying if the magnification factor automatic set key, a display for displaying if the magnification factor automatic selection mode is selected, a display for displaying if the paper size automatic selection mode is selected, a magnification factor automatic set unit for automatically setting a magnification factor when the magnification factor automatic mode is set, a paper size automatic selector for automatically selecting a paper size when the paper size automatic selection mode is set, a mode reset unit for resetting a proper mode when a manually input paper size and an automatically calculated paper size are both available or when a manually input magnification factor and an automatically calculated magnification factor are both available, and an automatic selection mode controller for generating an adjusted paper size or an adjusted magnification factor when a copy operation can be performed with the adjusted paper size or magnification factor even if a conflict has occurred.