In a detection method for detecting the quantity of a target material, a labeled binding material in the amount corresponding to the quantity of the target material contained in a liquid specimen is bonded to the top of a sensor portion; and a signal based on light emitted from a label in an evanescent field or an enhanced optical field produced on a surface of the sensor portion when the sensor portion is irradiated with excitation light is detected. After the labeled binding material is bonded to the immobilization layer, the signal is detected while the fluid over the sensor portion is controlled to flow at a constant flow rate at which bonds between the labeled binding material and the immobilization layer are not broken and the above signal can be detected with a greater magnitude than when the liquid specimen exists over the sensor portion at rest.