1. A processing unit among a plurality of processing units that form a distributed system, comprising: storage means for storing target knowledge with regard to a given target and problem solving knowledge representing how problem solving with regard to said target is to be performed; communication means for communicating with a mediation agent to transmit information of the processing unit and receive information from the mediation agent; and processing means for performing a part of said problem solving by making use of said target knowledge and said problem solving knowledge fundamentally and making use of said information from said mediation agent optionally, wherein said mediation agent is generated when the processing unit is required to cooperate with another processing unit, wherein the mediation agent migrates among said processing units and has cooperation problem solving knowledge that represents how said processing units cooperate with each other to perform said problem solving, and wherein the mediation agent mediates cooperation among said units by making use of said cooperation problem solving knowledge to cause said each unit to perform said problem solving.