The deformation of a thin viscoelastic plate is usually analyzed by use of an equation that is derived with an artificial assumption of the Poisson's ratio equal to 1/2. It is shown in this paper that a simple treatment application to an arbitrary value of Poisson's ratio is possible by introducing a new variable that represents the pressure gradient between the top and bottom of the plate. In this treatment a one-dimensional stationary deformation of moving plate is given by a simple analytical equation. This equation is used to analyze the bathymetry of the outer topographic rise seaward of the deep-sea trenches. The analysis reveals that the lithosphere in the western Pacific trenches is subject to an extensional horizontal force of 100 MPa or greater. The horizontal force may be much less or even compressive in the eastern Pacific trenches. To prevent horizontal force and bending stress from being extraordinarily large, the Maxwell relaxation time of the plate must be 1 m.y. or shorter, and the effective viscosity must be 1024 pa s or less.