To generate an optimum communication schedule when data is transmitted or received between processors which constitute a parallel computer or a distributed multiprocessor system.Processors which each perform inter-processor communication are sorted into a plurality of groups. A communication graph is generated whose nodes correspond to the groups and edges correspond to the communications. Communication graphs are generated for distances between nodes from one through N-1. Each communication graph corresponds to a communication step of the inter-processor communication. Communication is grasped as a whole by the communication graph and the edge of the communication graph means the inter-processor communication which is performed in a certain communication step. In this way, the communication can be optimized.