A threshold matrix generation part determines a turn-on order in a matrix area where a threshold matrix is generated and determines a threshold value of each matrix element. In determining the turn-on order, a temporary evaluation value element is obtained based on a distance between each undetermined matrix element whose position in the turn-on order has not been determined and each determined matrix element. If these matrix elements are located in the same position with respect to a row or column direction, the temporary evaluation value element is corrected such that the undetermined matrix element is less evaluated based on the evaluation value. Then, the position of an undetermined matrix element that is most highly evaluated in the turn-on order is determined. Accordingly, a threshold matrix that suppresses the occurrence of a grid-like dot pattern can be provided.