It is not easy to install a power generation system with renewable energy in the island such as wind power or solar power generation system because the space of the island area where the grid connection is difficult is limited. However, the island has a large ocean space, so it is easy to apply wave energy converter(WEC). Particularly, there is a breakwater in the island and it is economically advantageous to apply WEC to the breakwater. This study considers the basic design of an oscillating water column (OWC) and an impulse turbine of a small WEC system applicable to a breakwater in the island where the grid is not connected. Considering the connectivity of the breakwaters and the operability in the island area, an oscillating water column with a sloping shape is adopted. The basic design is carried out through potential flow analysis, CFD analysis, and model tests. The impulse turbine is adopted, which has been applied recently, and the basic design is evaluated through sensitivity analysis on design parameters. Based on these, a method for the basic design of a small OWC wave energy converter(WEC) applicable to breakwater is proposed and discuss what is important in the design of a small OWC wave energy converter.