To provide a digraph preparation method for preparing a digraph whose visibility is high with small processing quantity.

A descendant relation investigation procedure 112 prepares a descendant relation hash table 121 where nodes configuring a digraph are associated with the descendant nodes of those respective nodes. A hierarchy decision procedure 131 extracts the slave node of the node of the least significant hierarchy of the preliminarily decided hierarchies, and refers to the descendant relation hash table 121 to decide whether or not a hierarchy just below the least significant hierarchy of the already decided hierarchies can be decided to the extracted node, and decides the hierarchy to the slave node whose hierarchy decision possibility is decided.