© 2020 Elsevier B.V. Severe fever with thrombocytopenia syndrome (SFTS) is a life-threatening febrile illness that is caused by the SFTS virus (SFTSV). The diagnosis of SFTS is usually performed by detecting viral RNA. However, it has been reported that viral RNA is no longer detectable at 6–12 days after the onset of disease. In the current study, we have constructed a plasmid to express the recombinant nuclear protein (NP) based on the Japanese strain of SFTSV (J1). We developed a double-antigen enzyme-linked immunosorbent assay (ELISA) and immunochromatography (IC) assay using recombinant NP to detect antibody against SFTSV-NP. When we tested time-sequential samples from four patients with SFTS, antibody to SFTSV-NP were detectable not only during the recovery phase (days 10–622) but also during the acute phase (days 4–7) of the disease using both of a double-antigen ELISA and IC assay. SFTSV-RNA was detected until 8–11 days after onset, thus suggesting the coexistence of the virus and antibody during the acute phase of SFTS. These data suggest that assays for detecting antibody against SFTS-NP described in the current study may be applicable not only for the epidemiological studies but also for the diagnosis of SFTS.