1. A conversation control computer comprising a general-purpose computer for implementing a conversation controller and conversation control method according to a program therefor stored on a nontransitory physical storage medium operable with the general-purpose computer, the conversation control computer being configured to retrieve, based on input information received from a user, a reply sentence to the input information and 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 including a morpheme including a character, a string of characters or a combination thereof, and a plurality of reply sentences, which are associated with the pieces of second morpheme information; 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 including a portion of the first morpheme information from among the pieces of second morpheme information; 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 searched piece of second morpheme information; a topic identification information search unit configured to compare, based on the first morpheme information extracted at the morpheme extracting unit, the first morpheme information with pieces of topic identification information for identifying a topic, and to search a piece of topic identification information corresponding to the at least one morpheme constituting the first morpheme information from among the pieces of topic identification information, wherein the pieces of topic identification information are each associated with pieces of second morpheme information; the topic search unit is configured to compare, based on the searched piece of topic identification information, pieces of second morpheme information associated with the searched piece of topic identification information with the first morpheme information extracted at the morpheme extracting unit, and to search a piece of second morpheme information corresponding to the first morpheme information from among the pieces of second morpheme information associated with the searched piece of topic identification information; and an elliptical sentence supplementation unit configured to add the searched piece of topic identification information to the first morpheme information extracted at the morpheme extracting unit to provide a supplemented first morpheme information when no piece of second morpheme information including a portion of the extracted first morpheme information can be located by the search performed at the topic search unit, wherein, the topic search unit is configured to search, based on the supplemented first morpheme information, a piece of second morpheme information including a portion of the supplemented first morpheme information from among the pieces of second morpheme information.