Japanese encephalitis virus (JEV) is a flavivirus and is responsible for over 30,000 annual cases of encephalitis worldwide, with a mortality rate of approximately 30%. Therefore, it is important to check the distribution of JEV in fields, although recently, the number of Japanese encephalitis (JE) cases has been approximately five per year in Japan. We report the seasonal dynamics of mosquitoes between 2010 and 2014 in Ishikawa Prefecture, Japan. We collected 39,308 female adult mosquitoes, 98.2% of which were classified as Culex tritaeniorhynchus Giles. We identified JEV genomic RNA belonging to genotype 1 among the homogenate of Cx. tritaeniorhynchus collected during our study using reverse transcription-polymerase chain reaction and nucleotide sequencing methods. Our results indicated that mosquito vectors for JEV are distributed not only in areas in Ishikawa but also throughout Japan and suggest that we must be careful regarding JEV outbreaks in Japan in the future.