a memory configured to structurally store graph data that describes nodes representing a plurality of registered words, using edge information that indicates a mutual relationship; and

processing circuitry configured to automatically

extract a word from text information to be supplemented;

determine whether the word is a specific word needed to be supplemented on the basis of another registered word associated by the relationship information from a registered word corresponding to the extracted word among the plurality of registered words stored in the memory;

when the word is determined as the specific word, reference the memory and generate supplemental information for supplementing the word on the basis of the relationship information deriving from the registered word coinciding with the word and of another registered word,

store, in a conversation information database, text information being conversation information in response to speech information transmitted from a user terminal;

acquire speech information transmitted from a user terminal, acquire text information being conversation information in response to the speech information from the conversation information database, and transmit the information,

extract a word from the acquired text information, and

transmit the conversation information and the supplemental information to the user terminal.