In the case where many interactional substances are immobilized on the front surface side of a light transmitting plate capable of generating evanescent waves on the front surface, and where the fluorescence levels in the interactions between the interactional substances and a fluorescence labeled test substance are detected in a liquid layer according to a conventional method, the detection sensitivity and S/N ratio are low due to the background noise. Furthermore, it is necessary to perform a gel filtration purification step for the purpose of removing an excessive amount of the fluorescence dye used for labeling a test substance before the interactions between the fluorescence labeled test substance and the interactional substances are performed. To solve these problems, this invention proposes the above-mentioned method for detecting the evanescent wave excited fluorescence comprising the step of adding a black colorant, for example, a pigment based colorant mainly containing fine particles such as Bokuju.