Surface-enhanced Raman scattering (SERS) makes the Raman signals, as fingerprints of different vibration modes of chemical bonds, significant in practical applications. Two main mechanisms, chemical and physical, are attributed to the SERS of molecules adsorbed on metals. The physical mechanism plays a major role in SERS, which is the focus of our paper. Recent SERS systems are mostly based on dimer structures, i.e. nanoparticle pairs, of noble metals. Large amplification of electrical field occurs in the gap of a dimer structure compared with a single nanoparticle. The above gap positions are called as " hot spots” of SERS. In addition, the reproducibility and reliability of SERS substrates are also important for practical applications. Here we use periodical subwavelength metallic structures to meet such needs, and develop other kinds of electrical field enhancement mechanisms. We present the electrical field enhancement of the band-edge mode of surface plasmon polariton, gap plasmon polariton mode, as well as their coupling mode. We choose one-dimensional subwavelength metallic structures to clarify the physical mechanism. Our purpose is to develop subwavelength metallic structures with even and intensive " hot spots”, serving as ultrasensitive solid-state SERS substrates with excellent reproducibility and reliability.