Accurate measurement cannot be performed due to magnetic signals from magnetic impurities included in a specimen container. By finding a difference between two measurement signals obtained by applying an external magnetic field for orientation to respective normal and reverse directions, the magnetic signals from the magnetic impurities included in the specimen container which are not dependent on the external magnetic field for orientation can be cancelled. The influence of the magnetic impurities included in the specimen container is reduced, and the signal of the intended bound magnetic marker can be measured with high sensitivity.