In this paper, a self-reaction point absorber wave energy converter (WEC) is studied, where energy is collected by the relative motion of the on-board plate and buoy. The purpose of this study is to increase the relative displacement to obtain the maximum power. A semi-active inerter is employed, and dimensionless representation of the relative displacement is derived, Furthermore, the optimal inertance is also calculated. The motivation of employing semi-active inerter is to match with the wave frequency by changing the natural frequency. The effect of semi-active inerter on the plate is mainly focused on, and the results show that the power absorption is greatly improved by applying semi-active inerter both in regular and irregular waves.