1. A speech recognition method using machine learning, comprising: receiving a speech signal as an input, performing speech recognition to generate speech recognition result information including multiple candidate sentences and ranks of the respective candidate sentences; processing the multiple candidate sentences included in the speech recognition result information according to a machine learning model which is learned in advance and changing the ranks of the multiple candidate sentences to re-rank the multiple candidate sentences; and selecting a highest-rank candidate sentence among the re-ranked multiple candidate sentences as a speech recognition result, wherein the machine learning model is generated by: receiving the speech signal and a correct answer sentence as inputs; performing the speech recognition on the speech signal to generate the speech recognition result information including the multiple candidate sentences and sentence scores representing the ranks of the respective candidate sentences; adding the correct answer sentence to the speech recognition result information to generate a correct answer set; extracting features of the candidate sentences and the correct answer sentence included in the correct answer set to generate learning data; and performing the machine learning of changing the ranks of the candidate sentences according to differences between the features of the candidate sentences and the features of the correct answer sentence based on the learning data, and wherein the features include speech recognition ranks, a sentence score of the highest-rank candidate sentence, a morpheme bigram, a POS (part of speech) bigram, the number of domain dictionary unregistered words, morphemes/POSs of domain dictionary unregistered words, the number of general dictionary unregistered words, and morphemes/POSs of general dictionary unregistered words.