The present invention provides an immunoassay technique which enables efficient detection of antigen-antibody reaction with high sensitivity by a magnetic method using magnetic particles and a SQUID magnetic sensor or sensors. A system based on the technique includes a disk-shaped sample holder which holds on a circle a plurality of sample containers for accommodating marked samples, resulting from marking of samples with magnetic particles by antigen-antibody reaction; rotating means for rotating the holder around its central shaft; magnetizing means for magnetizing the marked samples outside a magnetic shield; and a magnetic sensor for detecting, within the magnetic shield, magnetic fields generated from the marked samples which have been magnetized. By rotation of the holder, areas fixing and holding different ones of the sample containers are successively inserted into the magnetic shield, and the magnetization of the marked samples accommodated in first ones of the sample containers and the detection of magnetic fields generated from the marked samples accommodated in second ones of the sample containers are executed in parallel.