© 2018 Elsevier B.V. Weekly field observations were conducted in Harima-Nada in the eastern part of the Seto Inland Sea, Japan from 21 August to 26 October 2015 to investigate the role of heterotrophic dinoflagellate Noctiluca scintillans on a coastal ecosystem. We observed a rapid N. scintillans growth just after increasing Chl. a, of which the dominant species were Chaetoceros spp., Thalassionema spp., Thalassiothrix spp., Coscinodiscus spp., Thalassiosira spp., and Stephanopyxis spp. We also observed under a microscope that N. scintillans cells grazed on these species of diatoms. The biomass of N. scintillans was estimated to increase from 4.50 to 470 mgC m−2 in 7 days. On the other hand, phytoplankton biomass drastically decreased from 2110 to 725 mgC m−2. The increase in N. scintillans biomass could account for 33.6% of the decrease in phytoplankton biomass. If we assume all the decrease in phytoplankton biomass was due to the N. scintillans grazing of diatoms, the 33.6% indicates the apparent gross growth efficiency of N. scintillans under a sufficient source of feed. Our results indicated the active feeding of N. scintillans on diatoms ends a diatom bloom in coastal water.