A magnetic immunoassay (MIA), which utilizes functionalized magnetic nanoparticles, is expected as a sensitive and rapid measurement tool for biological materials. In our group, MIA with high-TC superconducting quantum interference devices (HTS-SQUIDs) have been developed to realize the MIA system with the highest sensitivity. In addition, magnetic characteristics of magnetic nanoparticles which affect MIA is evaluated using HTS-SQUID based measurement system to realize highly sensitive MIA. We believe that our MIA system has excellent potential for early diagnosis of disease, contributing to improve our quality of life.