The present invention provides an image forming apparatus and a method of controlling an image forming apparatus. An image forming apparatus includes an operation display portion, an image reading portion which reads a reading region including an original document so as to generate image data and a control portion which generates basic display data for displaying an image in all parts of the reading region including an original document image. When the size of the original document is a threshold value or more smaller than the size of the reading region, the control portion generates enlargement display data for displaying the original document image whose display size is enlarged as compared with the original document image displayed based on the basic display data and makes the operationdisplay portion produce the display based on the enlargement display data.