A method, the method includes determining a target segment from a speech signal, determining a target character sequence corresponding to the target segment by decoding the target segment, identifying a first overlapping portion between the target character sequence and a preceding character sequence based on an edit distance, and merging the target character sequence and the preceding character sequence based on the first overlapping portion. A cost applied to the edit distance is determined based on any one or any combination of any two or more of a type of operation performed at the edit distance, whether characters to be operated are located in the first overlapping portion, and whether the characters to be operated match.