A machine learning method includes: obtaining first teacher data, which includes first encrypted words and corresponding search word information including one or more second encrypted words to be used for search, the first encrypted words being generated such that the first encrypted word includes a code sequence different from other encrypted words even though both of the first encrypted words and the other encrypted words have been generated from a same word; obtaining a group of words from among the first encrypted words by using a trapdoor scheme; generating second teacher data by using one encrypted word included in the obtained group to replace a rest of the obtained group of words; and performing, on the basis of the second teacher data, machine learning of a parameter to determine, in response to receiving of one or more encrypted words, one or more encrypted words to be used for search.