An optical detection method for a target substance that detects fluorescence or scattered light as an optical signal from a conjugate comprising the target substance on a surface of a sensing plate (2, 52) subjected to a near optical field, comprising: a conjugate moving step, and detecting the target substance by measuring a decrease in or a movement of the optical signal caused by movement before and after the conjugate moving step, wherein the conjugate comprises at least the target substance bonding with a magnetic particle, and wherein the surface is chemically treated to suppress adsorption of the conjugate and is capable of moving the conjugate presenting in the near optical field to outside of the near optical field according to an applied magnetic field, and the conjugate moving step is implemented by any one of the steps of: a first conjugate moving step of applying a first magnetic field such that the conjugate presenting in the near optical field moves to inside the near optical field or outside the near optical field in a direction parallel to the surface or away from the surface; and a second conjugate moving step of drawing the conjugate on the surface by applying a second magnetic field from a magnetic field application unit disposed on a side of a rear face of the sensing plate (2, 52), and moving the magnetic field application unit in a direction having a vector component parallel to an in-plane direction of the surface of the sensing plate (2, 52) while applying the second magnetic field so as to move the conjugate presenting in the near optical field in a state to be movable out of the near optical field to inside the near optical field or outside the near optical field in accordance with the movement of the magnetic field application unit. a conjugate moving step, and detecting the target substance by measuring a decrease in or a movement of the optical signal caused by movement before and after the conjugate moving step, wherein the conjugate comprises at least the target substance bonding with a magnetic particle, and wherein the surface is chemically treated to suppress adsorption of the conjugate and is capable of moving the conjugate presenting in the near optical field to outside of the near optical field according to an applied magnetic field, and the conjugate moving step is implemented by any one of the steps of: a first conjugate moving step of applying a first magnetic field such that the conjugate presenting in the near optical field moves to inside the near optical field or outside the near optical field in a direction parallel to the surface or away from the surface; and a second conjugate moving step of drawing the conjugate on the surface by applying a second magnetic field from a magnetic field application unit disposed on a side of a rear face of the sensing plate (2, 52), and moving the magnetic field application unit in a direction having a vector component parallel to an in-plane direction of the surface of the sensing plate (2, 52) while applying the second magnetic field so as to move the conjugate presenting in the near optical field in a state to be movable out of the near optical field to inside the near optical field or outside the near optical field in accordance with the movement of the magnetic field application unit.