Numerical simulation by a three-dimensional particle tracing method is applied to study on the tidal exchange of Hakata Bay, a semi-closed bay located in the northern part of Kyushu, western Japan, where an artificial island is being reclaimed and stagnation of the flow is a concern since this can lead to deterioration of the water quality inside the bay. The tidal current is calculated by a multi-level method and the movement of labeled particles is determined by the Eulerian-Lagrangian method. The characteristic of tidal exchange of the bay is evaluated by the remnant function defined by the ratio of particles remaining in the bay at each period to the initial number of particles. The average residence time is calculated by integrating the remnant function. Effect of the presence of the artificial island on the tidal exchange of the bay is studied. The construction of a new waterway through a sandbank is proposed and its effects on tidal exchange are also studied.