1. An inkjet printer, comprising: an inkjet head for forming dots having different sizes on a printing medium; a moving mechanism for moving said printing medium relatively to said inkjet head; a memory which stores a plurality of threshold matrix sets, a size of dot formed at each position on said printing medium being determined by comparing an original image with a threshold matrix set; and a controller for selecting a threshold matrix set in accordance with a classification of said original image and halftoning said original image; wherein each threshold matrix set included in said plurality of threshold matrix sets is a group of a plurality of threshold matrices corresponding to a plurality of sizes of dots, the largest size of formed dots becomes larger and an occupancy rate of dots having the largest size and an occupancy rate of all dots both increase in accordance with increase of gray level of said original image when comparing said original image with said each threshold matrix set, occupancy rates of all dots which correspond to a first threshold matrix set and a second threshold matrix set included in said plurality of threshold matrix sets, are different from each other in at least one part of a whole range of gray level and an occupancy rate of all dots corresponding to said first threshold matrix set becomes lower than or equal to that corresponding to said second threshold matrix set in a whole range of gray level.