1. A device for performing interaction between a user and a machine, the device having a plurality of domains corresponding to multiple phases of the interaction, each domain including speech understanding means for understanding content of a speech of the user and outputting a speech understanding result, the device comprising: means for recognizing the speech of the user from a signal detected by a microphone; means for delivering the speech of the user to the respective speech understanding means, receiving the speech understanding result from each of the speech understanding means and selecting, as a relevant domain, the domain having the speech understanding means that produced an optimum speech understanding result; means for referring to a task knowledge of the relevant domain out of task knowledges included in the respective multiple domains, and extracting a task associated with the speech understanding result; means for referring to a subtask knowledge including a plurality of subtasks associated with the kind of task, and determining a sequence of subtasks associated with the extracted task; means for determining a first subtask of the sequence of subtasks as a relevant subtask and updating, as the relevant domain, the domain to which the relevant subtask belongs; means for referring to an action knowledge of the relevant domain out of action knowledges included in the respective multiple domains, and extracting a subtask completion flag or an action associated with the speech understanding result and the subtask; and means for causing the machine to execute the extracted action.