A prohibition turn determination apparatus determines an initial path so that the communication amounts of respective links connecting switches to each other are most efficiently distributed between a communication pair, that is, a pair of servers communicating with each other via a network. Then, the prohibition turn determination apparatus calculates the communication amounts of respective turns formed along the initial path based on the communication amounts set between end nodes which are the communication pair. Next, the prohibition turn determination apparatus determines prohibition turns, which are not used for packet communication, based on the communication amounts of respective turns by an Up/down method or a TP method. Finally, the prohibition turn determination apparatus determines the final routing to avoid the prohibition turns.