© 2019 IEEE. This work describes simple method for sensing bacteria, Legionella by using a photogate-type optical sensor (PG-sensor) and MEMS chip. We observed the behavior of fluorescence from Legionella dumoffii (L. dumoffii) and Legionella erythra (L. erythra) when they are exposed to ultraviolet light. The time dependence of the fluorescence intensity of L. dumoffii and L. erythra obtained with PG-sensor agreed well with that of fluorescence intensity obtained with spectrometer, showing that each Legionella is able to be detected by PG-sensor. Using the difference of their behavior, the identification of each bacterium was successfully performed for a mixed bacterial system of L. dumoffii and L. erythra by using the combination of PG-sensor and optical filters.