An image forming apparatus includes image data memory for storing image data, an image displaying section for displaying an image on a CRT, a graphic data memory for storing graphic data corresponding to patterns for defining regions of interest in the image on the CRT and a synthesized data producing section for synthesizing the image data and the graphic data to produce synthesized data, the image displaying section being adapted to display the image and pattern on the CRT based on the synthesized data. The apparatus further includes a pattern selecting section for selecting graphic data corresponding to the patterns for defining the regions of interest from among the graphic data stored in the graphic data memory, an editing mode determining section for determining an editing mode for editing the pattern corresponding to the selected graphic data, a graphic data editing section for editing the selected graphic data based on the determined editing mode, and edited graphic data producing section for producing edited graphic data based on the result of the editing of the graphic data editing section, the synthesized data producing section being adapted to produce synthesized data by synthesizing the image data, the graphic data and/or the edited graphic data. According to the thus constituted apparatus, it is possible to surround an area in the image displayed on the CRT by a pattern to specify a region of interest in a desired manner with a memory having a small capacity.