Method for assisting in detection of breast cancer

Disclosed is a method for assisting in the detection of breast cancer, the method being for assisting in highly accurately detecting breast cancer. A method for assisting in the detection of breast cancer, the method using as an index the amount of at least one type of miRNA, isomiR, precursor miRNA, or transfer RNA fragment that has a specific base sequence that is present in a test sample that has been isolated from an organism. The following indicate a high likelihood of breast cancer: there being more of at least one type of miRNA or the like that is represented by any of SEQ ID NOs: 1–19, 27, 28, 34–51, or the like than in a healthy person; or there being less of at least one type of miRNA or the like that is represented by any of SEQ ID NOs: 20–26, 29–33, 52–54, or the like than in a healthy person.