An electric field distribution of a surface normal wave on a corrugated metal plate used for a surface wave oscillator was calculated. The surface wave oscillator is formed by the corrugated metal plate and a sheet electron beam. The direction of the beam propagation is parallel to the metal plate and perpendicular to the corrugation. In the vicinity of the sinusoidal corrugated metal plate, the electromagnetic wave of the surface normal mode which propagates parallel to the plate exists. From the surface normal mode computation, it was confirmed that an electric field distribution had a periodic component to the traveling direction of the beam. Cherenkov interaction should be excited by the electron beam passing in this periodic electric field region. The surface normal wave always exists only in a slow wave region, and has backward wave with the periodic boundary condition. This interaction becomes absolute instability, because the interaction is on the backward wave of the surface normal mode in the slow wave region. A strong oscillation by the surface normal wave should be generated, because the wave can generate the absolute instability of the Cherenkov interaction.