1. A conversation control system which retrieves, based on input information received from a user, a reply sentence to the input information, comprising: a morpheme extracting unit configured to extract, based on a character string corresponding to the input information, at least one morpheme constituting a minimum unit of the character string, as first morpheme information; a conversation database configured to store pieces of second morpheme information each showing a morpheme including a character, a string of characters or a combination thereof, and a plurality of reply sentences, which are associated with one another; a topic search unit configured to compare, based on the first morpheme information extracted at the morpheme extracting unit, the first morpheme information with the pieces of second morpheme information, and to search a piece of second morpheme information corresponding to the first morpheme information from among the pieces of second morpheme information; and a reply retrieval unit configured to retrieve, based on the piece of second morpheme information searched at the topic search unit, a reply sentence associated with the piece of second morpheme information.