No robot that can make a dialog customized for the user is known to date. According to the invention, various pieces of information appendant to an object are stored as values of the corresponding items of the object and a topic that is related to the topic used in the immediately preceding conversation is selected. Then, an acquisition conversation for acquiring the value of the item of the selected topic or a utilization conversation for utilizing the value of the item of the topic that is already stored is generated as the next conversation. The value acquired by the acquisition conversation is stored as the value of the corresponding item.