The invention relates to a method that detects the target substance by measuring a decrease in the optical signal generated by applying a first magnetic field to move the conjugate in the direction parallel to or away from the surface, etc. The invention also relates to an optical detection device wherein a liquid holding part has a sensing plate that can form a near field on a surface with light irradiated from a side of a rear face under a condition of total reflection, the liquid holding part being capable of holding a liquid sample that has a target substance and a magnetic particle that forms a conjugate with the target substance on the surface of the sensing plate, a light irradiation unit, a light detection unit, and a magnetic field application unit configured to apply a magnetic field to move the conjugate in the direction parallel to or away from the surface, etc.