1. 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 among combinations of a plurality of types of threshold matrix for performing binarization processing on a continuous tone image signal and at least one type of division information for classifying a whole range of threshold values covered by the types of threshold matrix into a plurality of divisions; a code assigning portion that respectively assigns code values, which are based on whether or not pixels of the continuous tone image signal belong to the plurality of divisions, to the two or more combinations; and a dot size determination portion that determines the dot sizes of the respective pixels on the basis of the two or more code values respectively corresponding to the two or more combinations.