BRCA1/2 genetic testing to use PARP inhibitor for breast cancer has a possibility of the "secondary finding" among the younger nonaffected family members of the patient, which turns them into at-risk for hereditary breast cancer. Proper understanding of the background of the hereditary cancer is now required for appropriate acceptance of the risk. Therefore, we investigated the level of knowledge and attitudes of younger women on hereditary breast cancer in Japan. Study subject was Japanese university women between 20 and 30 years of age, without medical history of breast cancer. We conducted the anonymous self-answering questionnaire to them. We received responses from 353 women. The levels of knowledge, awareness, and interest were relatively high. Women with a family history of breast cancer were less likely to undergo testing than women without (92.8% vs. 74.5%, p < 0.001). The rates of positive response toward risk-reducing mastectomy (RRM) and risk-reducing salpingo-oophorectomy (RRSO) was significantly high for medical majors compared with that for other majors (RRM: medical 71.6% vs. science 54.5% vs. humanities 53.8%, p = 0.008, RRSO: 35.4% vs. 36.3% vs. 48.4%, p = 0.027). Approximately half of respondents answered that they would hesitate to get married (45.3%) or to have children (55.4%), if they were a BRCA1/2 mutation carrier. The results may help to establish the methods for supporting the decision-making for reproduction of younger women who are unexpectedly labeled as being at-risk for HBOC.