Orographical effects of a slope behind a flat plain on the land and sea breeze circulation are investigated using a numerical model with a simplified configuration of the lower boundary. It is found that (1) when the temperature at the slope surface varies with a diurnal period similarly to that at the plain surface, both the sea breeze and the land breeze are amplified and the alternation of the sea and the land breezes occurs earlier than the land and the sea breezes over a flat plain, and (2) when the slope works merely as a barrier, both the land and the sea breezes are reduced and the circulation domain is confined in the sea and the plain regions. The solution of linear differential equations for the unsteady slope wind is examined to make more clear the physical process in the above mentioned situation. The land and sea breeze in the former case is found to be modified by the slope wind which is stronger than and is in advance of the land and sea breeze over a flat plain. The land and sea breeze for the case with a slope varying its surface temperature is highly efficient in producing the available potential energy and converting it to the kinetic energy of the breeze.