Wave energy is attracting a lot of attention as a form of renewable energy and various studies are conducted around the world for the practical application of wave power converters. Among the various methods of wave power generation devices proposed so far, a point absorber wave energy converter (WEC) is more efficient and easier to install than the others. A WEC with a tuned inerter, which consists of a tuning spring and a rotational inertial mass, was proposed by the authors to enhance the power generation performance by taking advantage of the resonance effect. However, the frequencies of ocean waves vary from hour to hour in real sea state thus a mechanism to adjust the change of wave frequencies is required to improve the power generation performance. Therefore, in this research, a WEC with a variable tuned inerter, which can change the inerter value, is proposed. Also, to detect the dominant wave frequency, the Fast Fourier Transform (FFT) technique is applied. Then numerical simulation studies are carried out on a point absorber WEC using the one-day wave height data measured off the coast of Fukushima-Ken and in Kashima port. The results show that the proposed system with the FFT can improve the power generation performance.