The oscillating modes of thin elastic shells under unsteady fluid forces are numerically analyzed. The governing equations used in this paper are the Navier-Stokes equations, continuity equation and ones of motion of thin elastic shells. The spatial discretization of each equation is achieved with the finite element method. In the proposed numerical method, the finite element equations for incompressible viscous flows and shells are solved simultaneously at each time step of numerical simulation. Consequently, both the flow profiles around the shells and oscillating modes of ones can be obtained. This method is applied to numerical calculation of a coupled problem of an incompressible viscous flow and an elastic cylindrical shell in order to show the validity of our procedure.