Wind power generation is growing rapidly and is acting a main role for preventing global warming. Yet the number of wind turbine failures due to severe weather conditions in Japan has been increasing. The foremost cause for this increase in the failure rate of wind turbines has been lightning. The coastal area of the Japan Sea in the Hokuriku region are prone to "Winter Lightning," a form of high-energy lightning which is rare in most other parts of the world. Winter lightning damages many wind turbines in Japan every year. Mitsubishi Heavy Industries, Ltd. (MHI) has observed and measured "Winter lightning" which strike actual wind turbines in Japan since 2002. By doing so, the company hopes to establish a technology for rapidly releasing the lightning energy to the ground whenever lightning strikes. In this paper, the authors analyze lightning current measurements recorded by wind turbines stricken by lightning in various regions of Japan. Observed data reveals that IEC61400-24 protection level 1 is not sufficient for "Winter lightning." These results confirm the necessity of Japanese national research project "Guideline for Wind Turbines in Japan."