In this paper, offshore wind resources within the Japan’s EEZ (Exclusive Economic Zone) are assessed using wind speed data from the microwave scatterometer SeaWinds onboard QuikSCAT. At first, from the 10m-height wind speed from QuikSCAT, 60 m-height wind speed is estimated by using an empirical equation for height correction. Based on the 60m-height wind speeds, annual energy production is calculated under an assumption of installing 2 MW wind turbines every 0.64 km2. The annual energy production is then accumulated for the entire Japan’s territorial waters and EEZ (4.47×106 km2). As a result, it is shown that the total energy production is estimated to be 4.86×104 TWh/yr. This offshore wind energy potential within the EEZ is approximately 50 times higher than the actual annual electricity production in Japan.