The purpose of this investigation is to show how much is the three-dimensional effects of the hydrodynamic forces acting on a box-type vessel which is floating in the sea of finite depth and whose length-breadth ratio is 1. Making use of the characteristics of the body shape, the three-dimensional hydrodynamic forces on the body are calculated by the “joining method of divided region” which is originated by Miles and Gilbert and developed by Garrett et al. In the present paper, the calculation results of the exciting forces acting on the vessel by above method are presented and compared with those by Strip method and the experimental results. The three-dimensional calculation gives fairly good agreement with the experimental results, and there are some differences between the results by this method and those by Strip method.