1. A signal-processing apparatus comprising: a candidate-detecting unit which receives an input signal part from an input signal in time-divided fashion, and identifies characteristic patterns in a portion of the input signal part indicating the probability that the input signal part is a candidate part; a characteristic-extracting unit which extracts characteristic patterns from the input signal part which alone indicate the probability that the input signal part is the candidate part; and a detecting unit which detects whether the input signal part is the candidate part based on the characteristic data extracted by the characteristic-extracting unit; and a determining unit which determines whether the candidate part resulting from the extraction and detection performed by the characteristic-extracting unit and the detecting unit is a commercial message, wherein, the determining unit determines whether the candidate part is a commercial message by first applying a minimum-length priority rule and then applying an adjacent-candidate priority rule based on the result of the minimum-length priority rule and then applying a score priority rule based on the result of die adjacent-candidate priority rule.