An image formation device that forms an image which is formed of dots with a plurality of dot sizes, the image formation device comprising: a combination selection portion that selects two or more combinations (M1, P1; M2, P2..) among combinations of a plurality of threshold matrices having different dot distribution characteristics for performing binarization processing on an input continuous tone image signal and at least one division information for classifying a whole range of threshold values covered by each threshold matrix into a plurality of divisions according to boundary values corresponding to one or more of the threshold values of the respective threshold matrix, the divisions defining where the threshold values of the respective type of threshold matrix are located, each selected combination consisting of a threshold matrix and a division information; a code assigning portion that respectively assigns code values to respective pixels of the continuous tone image signal for each of two or more selected combinations by performing binarization processing of the continuous tone image signal using each threshold matrix and determining to which division the threshold values corresponding to the on-state pixels after binarization belong, the code values indicating for each combination to which division the respective pixels belong; a dot size determination portion that combines the code values for each pixel by concatenation in accordance with combination rules into combination codes and determines the dot sizes of the respective pixels on the basis of the combination codes, and a data storage portion that stores size correspondence information which indicates a correspondence relationship between the dot sizes and the combination codes, wherein the dot size determination portion determines the dot sizes by referring to the size correspondence information which is stored by the data storage portion. a combination selection portion that selects two or more combinations (M1, P1; M2, P2..) among combinations of a plurality of threshold matrices having different dot distribution characteristics for performing binarization processing on an input continuous tone image signal and at least one division information for classifying a whole range of threshold values covered by each threshold matrix into a plurality of divisions according to boundary values corresponding to one or more of the threshold values of the respective threshold matrix, the divisions defining where the threshold values of the respective type of threshold matrix are located, each selected combination consisting of a threshold matrix and a division information; a code assigning portion that respectively assigns code values to respective pixels of the continuous tone image signal for each of two or more selected combinations by performing binarization processing of the continuous tone image signal using each threshold matrix and determining to which division the threshold values corresponding to the on-state pixels after binarization belong, the code values indicating for each combination to which division the respective pixels belong; a dot size determination portion that combines the code values for each pixel by concatenation in accordance with combination rules into combination codes and determines the dot sizes of the respective pixels on the basis of the combination codes, and a data storage portion that stores size correspondence information which indicates a correspondence relationship between the dot sizes and the combination codes, wherein the dot size determination portion determines the dot sizes by referring to the size correspondence information which is stored by the data storage portion.