The procedure of calculations by the use of pulsating pressure distributions on a free surface is applied to three dimensional motions of a box-shaped floating structure of shallow draft in regular waves. Integral equations are solved numerically, and added mass, damping coefficient and wave exciting force are computed. The dependence of them on various length-to-beam ratios is shown. Motions and hydrodynamic pressures in regular waves are also computed, and the results of the computation are compared with those of the experiment in which a model of the box-shaped structure, corresponding to a floating dock, responds freely to the waves. It is observed that the computed values are in excellent agreement with the measured ones.