1. A threshold matrix generation method of generating a threshold matrix which is compared with a grayscale original image in halftoning said original image, comprising the steps of: a) preparing a plurality of types of basic tiles each of which is an arrangement of elements in two rows and two columns consisting of two ON elements each representing existence of a dot and two OFF elements each representing absence of a dot, and arranging a plurality of basic tiles each of which is one of said plurality of types of basic tiles, in a matrix area where a threshold matrix is generated, in accordance with a prohibition condition for at least prohibiting an arrangement of only ON elements or OFF elements in an area of two rows and two columns, to acquire an element matrix representing an arrangement of dots in a gray level of 50%; b) setting a dot at arbitrary one element on a highlight dot pattern which is a set of ON elements in said element matrix; c) repeating a process of adding a new dot to an element on said highlight dot pattern, said new dot being farthest from existing dots on said highlight dot pattern, on the premise of spatial repetition of said threshold matrix in halftoning of an original image, or repeating said process in accordance with a predetermined condition, to determine a turn-on order in which dots are added to elements in accordance with increase in gray level on a highlight side; d) setting dots at all elements excluding arbitrary one element on a shadow dot pattern which is a set of OFF elements in said element matrix; e) repeating a process of removing a dot of an element on said shadow dot pattern, said dot being farthest from elements in each of which a dot is not set on said shadow dot pattern, on the premise of spatial repetition of said threshold matrix, to determine a turn-off order in which dots are removed from elements in accordance with decrease in gray level on a shadow side; and f) determining a threshold value of each element in said matrix area in accordance with said turn-on order and said turn-off order.