Construction of a 7 km long sea dike was completed in 1997 to reclaim the inner part of the Isahaya Bay in the Ariake Sea, Kyushu, Japan. According to tidal analyses, tidal range after the dike construction is reduced by about 3 % in front of the dike and 1.4 % at the entrance of the Ariake Sea. This reduction is claimed as one of the main reasons for environmental changes in the sea. Two possible causes were suggested on the tidal modification; inner effect of the dike construction and outer effect of amplitude variation at the entrance of the sea. However, their relative importance is still debated. Using one-dimensional analytical model, we examined tidal modification mechanism by the dike construction and compared the inner and outer effects. The Ariake Sea is simplified as an enclosed channel with a branch of the Isahaya Bay. The model shows that shortening of the branch causes the reduction in tidal range in the entire channel. The dike construction can explains for more than 50 % of the tidal range in front of the Isahaya Bay, although the inner and outer effects are not decomposed at the channel entrance.