Three-dimensional wave deformation due to a horizontally submerged thin plate is analyzed by the Doublet Distribution Method (DDM). The plate is assumed to be located at a certain depth from sea surface in deep water. The linear wave diffraction theory is employed in this method. First of all, the influence of mesh refinement in the present analysis to the numerical results is checked by employing a submerged square plate model. Next, as a particular application example, a wave focusing phenomenon by a submerged convex-shaped plate model is examined by the present method. In addition, some hydraulic model tests are performed to verify the numerical results. The following findings and conclusions are obtained in the present investigation. (1) The present DDM can solve effectively the three-dimensional wave deformation phenomenon due to a horizontally submerged thin plate. Element size is the most important factor in the present analysis in order to obtain a good numerical result. It is found that the element size should satisfy the following criteria : B/b >7 as well as λ/ b >7, where the symbols b, B and λ denote an element size, a chracteristic length of the submerged plate and an incident wave length, respectively. (2) A wave focusing and the corresponding wave amplifying phenomena due to a submerged convex-shaped plate can be identified in both theoretical and experimental results. In almost all cases, a significant wave amplification phenomenon can be observed on the surface of plate. The maximum amplification factor (the ratio between the observed and incident wave heights) obtained has the value of about 3.0 at the edge of the plate. It is also observed that the wave amplified zone is widened in the shore-side area behind the plate as the wave period becomes shorter and the plate is submerged in shallower water. As a result, it can be stated that the submerged plate model studied herein would be an important structual element for controlling the natural wave field in the future, for example, for extracting electric energy from wave motion.