The present invention provides a method for diagnosing hepatitis C virus (HCV) infection, a preventive agent for and a prevention method for HCV infection, a method for treating HCV infection, and a method for screening anti-HCV drug candidates. The HCV diagnostic method of the present invention comprises the detection of HCV proteins present in leukocytes. The preventive agent for HCV infection comprises an antibody against HCV proteins as an active component. The method for treating HCV infection comprises performing leukopheresis or reduction therapy on an HCV-infected patient. Further, the method for screening anti-HCV drug candidates comprises selecting, from test substances, a substance that is able to inhibit HCV infection in monocytes or is able to reduce the amount of HCV antigens to HCV-infected monocytes.
