1. A dialog server comprising: at least one processor; a memory, coupled to said at least one processor; a persistent storage device, accessible to said memory and said at least one processor; a position storage module, stored in a non-transitory manner in said persistent storage device, which, when loaded into said memory causes said at least one processor to store, in at least one of said memory and said persistent storage device, positional information on at least a first avatar and a second avatar; an utterance receiver module, stored in a non-transitory manner in said persistent storage device, which, when loaded into said memory causes said at least one processor to receive (i) at least one utterance from said first avatar, and (ii) at least one utterance strength representing an importance or attention level of said at least one utterance; an interest level calculator module, stored in a non-transitory manner in said persistent storage device, which, when loaded into said memory causes said at least one processor to calculate at least one interest level between said first avatar and said second avatar, based on said positional information; a message processor module, stored in a non-transitory manner in said persistent storage device, which, when loaded into said memory causes said at least one processor to generate a message from said at least one utterance in accordance with a value calculated from said at least one interest level and said at least one utterance strength; and a message transmitter module, stored in a non-transitory manner in said persistent storage device, which, when loaded into said memory causes said at least one processor to transmit said message to said second avatar; wherein said message processor module generates the message from said at least one utterance only when a value calculated from said at least one interest level and said at least one utterance strength is not less than a predetermined threshold value; and wherein said interest level calculator module calculates said at least one interest level by: calculating cosine of an angle between a facing direction of said first avatar and a line connecting said first and second avatars; dividing said cosine by a distance between said first and second avatars; and multiplying said cosine by a normalization factor which satisfies a requirement such that a sum of all interest levels between said first avatar and a number of avatars in a circle having a predetermined radius and centered about said first avatar is unity.
5. The dialog server according to claim 1 , wherein said interest level calculator module calculates said at least one interest level from said positional information, such that said at least one interest level becomes higher as said first avatar and said second avatar become closer.