The problem of determining the complex natural frequencies of closed objects with arbitrary shape is considered. The surface of the complex body, which is supposed to have an infinite conductivity, is divided into a large number of plane patches (finite elements) where the current is assumed to be constant. The appropriate boundary condition for the electric field on the surface of the complex body is imposed, yielding a homogeneous system of equations for the current coefficients. The regions of resonance (natural frequencies) of the complex object are determined as those where the determinant of this system is close to zero. As an example, the complex natural frequencies of a rectangular parallelpiped (or a cube) are calculated.