To provide a message sending function which reflects "taste" in a destination agent designated by a user and a policy of the market in which agents operate. When multicasting a message to agents, a message monitor executes the steps of: receiving preferential destination information designated by a user along with the message; determining to which agents the message is to be sent, by referring to the preferential destination information; and sending the message to the agents determined as destinations. This preferential destination information designated by a user is the information representing a user's "taste" which is represented, for instance, by an agent name and a priority (or weight). It is also possible that the destination determining step comprises a step of referring to a messaging policy data defining priorities of agents to which the message can be sent. By doing so, it becomes possible to reflect a policy of a market's sponsor on message sending. Moreover, it is also possible to transform it so that a message destination is determined by referring only to a messaging policy data rather than preferential destination information designated by a user.