1. A method of recognizing speech by measuring confidence levels of respective frames, the method comprising: (a) obtaining frequency features of a received speech signal for the respective frames having a predetermined length; (b) calculating a keyword model-based likelihood and a filler model-based likelihood for each of the frames; (c) calculating a confidence score based on the two types of likelihoods; and (d) deciding whether the received speech signal corresponds to a keyword or a non-keyword based on the confidence scores.