A vector integral-differential equation to describe electromagnetic waves propagation in nanowaveguides and photonic crystals containing thin metal layers is developed. Exact solution of the equation is obtained with Galerkin method taking into account complex dielectric permittivity of metals in optical range. A simple method to find complex propagation constant for low-loss waveguide structures is developed and proved. Surface plasmon-polariton (SPP) waves were simulated in the structures under consideration.