We analyzed the virus shedding of an oseltamivir-treated patient who had been infected with the pandemic swine-origin influenza A (H1N1) virus which had an oseltamivir-sensitive neuraminidase. The virus was isolated from the pharyngeal swabs of the patient using MDCK cells, and the virus genome RNA was detected in the same samples both by real-time RT-PCR and RT-PCR. The virus was isolated until 44 h after oseltamivir administration although the virus genome was detected until one day after oseltamivir treatment was stopped. Due to their high sensitivity, RT-PCR and real-time RT-PCR may cause misdiagnosis by detection of viral genome which does not infect, and classical virus isolation and clinical symptoms are recommended for the evaluation of oseltamivir treatment.