A magnetic binding substance, which is a first binding substance that specifically binds with a target substance, having magnet enveloping dielectric particles, which have magnetic particles enveloped therein and surfaces modified with functional groups that exhibit polarity within a liquid sample, attached thereto, and a labeling binding substance, which is a second binding substance that specifically binds with the target substance having photoresponsive labels attached thereto, are mixed with the liquid sample such that binding reactions occur. A magnetic field is generated within a sample cell, to draw the magnetic binding substance to a local region. Excitation light is irradiated only onto a predetermined region including the local region while the magnetic binding substance is drawn to the local region, causing the photoresponsive labels present therein to generate optical signals. The optical signals are detected.