This paper proposes a new method of estimating incident wave state. This method measures output currents from point absorber type wave energy converter and estimates state of the incident wave such as wave amplitude and wave period. The wave period and height are significant information to maximize the output power or the energy conversion efficiency. However, additional sensors are deployed to measure those information and it always involves cost and maintanance issues. This can be estimated using extended Kalman filter (EKF) by measuring generator output currents in α - β domain which does not require any other equipments. The simplified dynamic equation including the point absorber floater and linear generator inside the floater are derived since the Jacobian matrix of the system is required to perform the EKF algorithm. Finally a numerical example is provided to shoe the effectiveness of the proposed estimator.