1. An ink jet printing system which uses a print head having a plurality of ink ejecting nozzles to form an image on a print medium that is moved relative to the print head; the ink jet printing system comprising: means for setting one of plurality of print modes; means for converting multivalued image data into grayscale data of a predetermined resolution and level according to the set print mode; means for selecting one of index patterns according to the print mode and the grayscale data, each of the index patterns having a plurality of sectioned areas corresponding to one pixel of a printing resolution of the print head, the areas being arrayed in at least the direction of relative movement, and predetermined print/non-print binary data being set in each of the sectioned areas to represent a density in an area corresponding to one pixel of the predetermined resolution; and means for ejecting ink from the nozzles toward the print medium according to the index pattern selected by the selection means; wherein the ink ejected from the nozzles is divided into a main droplet and a sub droplet following the main droplet; wherein the number of sectioned areas arrayed in the index pattern in the direction of relative movement is so determined that the main droplet and the sub droplet land on the print medium, spaced a distance almost equal to an integer times a width, in the direction of relative movement, of the area corresponding to one pixel of the predetermined resolution.