An image processing apparatus which can be operated using a small memory capacity and at a high speed. A screen memory for storing X and Y coordinate data for edge points of polygons and a sort memory for storing polygon numbers are provided. A polygon side computing unit computes data for sides of each of the polygons corresponding to each of the polygon numbers. A side pair computing unit computes side pair data in accordance with the data of sides of polygons, the side pair data comprising data of a pair of sides located on the scan lines of the screen. An inclination computing unit computes an inclination of each side of the side pair. A side pair memory stores inclination data and start and end point data of each side of the side pair, each data being stored in a position having a predetermined address. A frame memory stores data for the address of the side pair memory. An interpolation computing unit computes right and left intersection data on each of the scan lines. A processing unit processes the right and left intersection data so as to obtain dot data for the polygons, and outputs the dot data by synchronizing the dot data with the scan lines of the screen on the display unit.