Background: Laninamivir octanoate hydrate (laninamivir) is a commonly used drug for influenza virus. This study is part of our long-term, systematic surveillance to monitor its effectiveness. Methods: We investigated the duration of fever and other symptoms after laninamivir inhalation for the outpatients in the Japanese 2017/18 and 2018/19 influenza seasons, then compared the results with those of the previous six seasons. Results: The number of laboratory confirmed patients analyzed was 111 in the 2017/18 season and 84 in 2018/19. The median duration of fever for B was significantly longer than for A in the 2017/18 season (p = 0.0182). In the 2018/19 season, when we could compare only between A subtypes, the median duration of fever was significantly longer for A (H3N2) (p = 0.0290). In contrast, the differences in the median duration of other symptoms were not significant. With the previous six seasons added, we evaluated the data of 1,473 patients over eight sequential influenza seasons. Seasonal differences were observed in the prevailing types/subtypes. The median duration of fever among the types/subtypes was significantly different in some seasons, and it was generally longer for B than for A. The difference in the duration of fever among the eight seasons was all within 24 hrs, and it was statistically significant only for A (H3N2). Conclusions: These results indicate the continuing clinical effectiveness of laninamivir against all types/subtypes of influenza virus, despite its broad use in Japan. Longer duration of fever for B than for A was observed after laninamivir inhalation.