The high fatality rate currently associated with severe fever with thrombocytopenia syndrome（SFTS）in Japan suggests that some mild cases of SFTS have been underdiagnosed. To find mild cases of SFTS in Japan, we retrospectively investigated the status of anti-SFTS virus antibody in hospitalized patients from January 2008 to January 2013, who presented with high fever and one or more characteristics of thrombocytopenia, leukocytopenia, and elevation of liver transaminase. Anti-SFTS virus antibodies in their convalescent period were analyzed using an indirect fluorescent assay, an enzyme-linked immunosorbent assay, and a plaque reduction neutralization test. Nine patients, in the age group of 20?80 year-old were investigated（20’s = 2, 30’s = 3, 40’s = 1, 50’s = 1, 60’s = 1, and 80’s = 1）．Blood samples of the patients were taken 5 to 66 months after the onset of illness（the median was 12 months）．All the patients tested negative for anti-SFTS virus antibody. Although we could not confirm SFTS in any of the patients in this study, attempts to investigate patients with mild SFTS-like symptoms is necessary to detect the presence of mild cases of SFTS and to promote further understanding of the epidemiology of SFTS in Japan.