The possibility of surface waves amplification by direct electrical current is theoretically investigated in three-layered
system with periodical structure at the surface. The analytical calculations were performed in the frame of the Green
function method on the base of the concept of effective susceptibility. Numerical analysis clearly proves the presence of
convective instability and it means that such a system can be considered as a surface waves' amplifier. Because of
periodical structure the phase synchronism domain lies in the range of entirely accessible in practice values of wave
vector and frequency. The increment of spatial growth amounts to several tens within the region of phase synchronism. It
means that predicted effect of amplification can be applied on practice.