receiving the speech input;

obtaining one or more text candidates comprising a character string in which it is predicted that the speech input is to be converted by recognizing the speech input using an automatic speech recognition (ASR) model;

calculating a similarity between the speech input and each of a plurality of speech signals previously stored in a database;

identifying a speech signal for which the calculated similarity exceeds a predetermined threshold, from among the plurality of speech signals;

extracting, from the database, text history information corresponding to the identified speech signal from among a plurality of text history information previously stored in the database; and

performing training to adjust a weight of each of the one or more text candidates using the extracted text history information.