West Nile virus (WNV) is a mosquito-borne flavivirus that invaded New York City in 1999 and quickly spread and settled in all 48 contiguous United States and all 10 Canadian provin- ces. WNV is still expanding its territory and is now headed towards Latin American coun- tries. Neuroinvasive disease caused by WNV develops in less than 1% of the affected popula- tion, but its mortality rate is approximately 10%. An imported WNV infection case occurred in Japan in 2005. Reservoir birds and mosquito vectors for WNV infection cycle are existing in Japan. Thus, we need to be cautious for a possible invasion of WNV in Japan.