In this paper a numerical method of elastic stress analysis is proposed which will be applicable to various structures composed of thin plates. Generally speaking, analytical method is not suitable to stress analysis of a finite plate. In the case that interaction of plates composing an actual structure is studied, numerical method which is powerful for analysis of finite plate should be applied, since the analytical solution of an infinite plate is considered to be meaningless. In this case the reaction force which acts to one plate from another plates can be involved in the equilibrium equation of the plate as a term of body force, and for each plate this partial differential equation is solved numerically using iterative method to coincide reaction forces and displacements in each plate on the connecting line of plates. As an example the stress distributions in a stiffened plate with a side notch are investigated. By changing width of stiffener and length of slit notch, this example problem is solved numerically in this way. In several cases of slit notch, the distributions of stresses calculated numerically are compared with the experimental results.