In cancer screening, it is essential to undertake effective screening with appropriate methodology, which should be supported by evidence of a reduced mortality rate. At present, mammography is the only method for breast cancer screening with such evidence. However, mammography does not achieve sufficient accuracy in breasts with high density at ages below 50. Although ultrasonography achieves better accuracy in Breast Cancer detection even in dense breasts, the effectiveness has not been verified. We have planned a randomized controlled trial to assess the effectiveness of ultrasonography in women aged 40-49, with a design to study 50,000 women with mammography and ultrasonography (intervention group), and 50,000 controls with mammography only (control group). The participants are scheduled to take second round screening with the same modality 2 years on. The primary endpoints are sensitivity and specificity, and the secondary endpoint is the rate of advanced breast cancers.