Abstract: The present invention provides a determination method, a fluorescence measurement device, and a test agent, each of which can easily determine a genotype of a microorganism responsible for causing a periodontal disease, based on enzyme activity. The determination method of determining a genotype of the microorganism responsible for periodontal disease includes the steps of: irradiating a liquid sample with excitation light, the liquid sample including a bacterial body or a bacterial body-based extractive matter of the microorganism responsible for periodontal disease and a reagent in which a substrate for an enzyme reaction by the microorganism responsible for periodontal disease is fluorescently labeled, the liquid sample having a pH value thereof having been adjusted to not lower than pH 7.0 and not higher than pH 8.5 and then having been subjected to the enzyme reaction; and determining the genotype based on an intensity of fluorescence emitted from the liquid sample.