Four years after the construction of an artificial tidal flat in Osaka Bay, Japan, the food web structure was analyzed using stable isotope ratios of nitrogen and carbon. The nitrogen stable isotope ratios (δ15N) ranged from 8.8±0.3‰ in Chondrus giganteus f. flabellatus (macroalga) to 15.2‰ in Repomucenus beniteguri (fish), whereas the carbon stable isotope ratios (δ13C) ranged from -18.6±0.2‰ in POM (particulate organic matter) to -10.5±1.1‰ in Ulva sp. (macroalga). The food web structure of the artificial tidal flat was estimated to be composed of three trophic levels. Moreover, it was thought that fishes inhabiting this ecosystem are dependent on POM and macroalgae (and/or benthic microalgae) for their carbon source, whereas crustaceans and mollusks are likely to be dependent on macroalgae (and/or benthic microalgae), not on POM. These results indicate that the food web structure of the artificial tidal flat resembles those of natural estuaries along the Japanese coasts. A temporal analysis of the food web structure of an artificial tidal flat and a comparison of such a structure to that of closely situated natural tidal flats using stable isotope analysis will contribute significantly to the evaluation of the structure and function of artificial tidal flats.