Lacustrine blooms, extraordinary growth of cyanobacteria, contribute to the deterioration of the water quality of dam-formed lakes. It is, therefore, necessary to establish a method for monitoring blooms both spatially and temporally. In this study, we used an acoustic method to investigate the relationship between acoustic reflection intensity and the number of cells. The study was carried out at the Miharu dam reservoir (Lake Sakura), Tamura-gun, Fukushima, Japan, in August 2019 and 2020. Using a 120 kHz quantitative echo sounder, volume backscattering strength (Sv) was measured to 10 m from the surface, where blue-green algae move vertically. Cell numbers of plankton in samples were counted. As a result, the Sv at these depths ranged from –81 to –55 dB. The cyanobacteria count ranged from 11,000 to 232,362,600 cells/l, and the most abundant species was Microcystis aeruginosa . There was a significant correlation between Sv and the number of cyanobacteria cells ( p < 0.01). Because gases from the air contributed to the acoustic reflection intensity, it is believed that the gas vesicles in M. aeruginosa also contributed to the acoustic reflectance and therefore to the significant correlation between Sv and the number of cyanobacteria cells. In the future, it is expected that the acoustic measurement method will be valuable for evaluating cyanobacteria spatially and temporally in the Miharu dam reservoir.