An iterative method is proposed to solve integral equations (IEs) of the second kind with Picard-kernel in linear complexity, i.e.O(N). The particular IE considered describes the process of scattering of a plane wave incident on an inhomogeneous slab. The collocation method with triangle basis functions is used to derive a linear system of equations, which is solved for a test problem with the BiCGSTAB method. To reduce the number of iterations, an efficient preconditioning operator is introduced.