Mesocosm experiment facility for coastal tidal flats was constructed in the laboratory of Port & Harbour Research Institute, Ministry of Transport. This facility has three tidal flats, surface area of which is 15-20m2 each. Dry sediment was transported from natural tidal flat in tokyo Bay. Sediment was placed in the facility with 50cm thick. Natural untreated seawater was introduced to the facility to be given semi-diurnal tidal fluctuation of 70cm in 11 hours. Operation started from Jan., 1995. Initially, no benthic biota was found. No transplantation nor input of benthic animals were made by human hands at all. Two years' monitoring shows that within 8 months initial intrusion of benthic biota was finished. After that, succession of benthic pieces started.