A method for replication of hepatitis C virus genome comprising infecting animal cells that have been infected with mouse retrovirus, with hepatitis C virus, and then culturing those cells. The present invention further provides a method for distinguishing a hepatitis C virus with high infectivity and a hepatitis C virus with less infectivity, comprising centrifugating a sample comprising a virus to be tested in a liquid medium to determine the density of the virus, and assigning the virus to a hepatitis C virus with high infectivity if the density thereof is up to about 1.06 g/ml, and assigning the virus to a hepatitis C virus with less infectivity if the density thereof is at least 1.13 g/ml; a method for distinguishing a hepatitis C virus with high infectivity and a hepatitis C virus with less infectivity, comprising brining a virus to be tested into contact with an anti-human immunoglobulin, and assigning the virus to hepatitis C virus with high infectivity if it does not form an immunocomplex and assigning the virus to a hepatitis C virus with less infectivity if it does form an immunocomplex; and a method for distinguishing a hepatitis C virus with high infectivity and a hepatitis C virus with less infectivity, comprising inoculating a virus to be tested to cells infected with murine retrovirus, culturing the cells, and assigning the virus to a hepatitis C virus with high infectivity if the virus replicates in the cells, or assigning the virus to a hepatitis C virus with less infectivity if the virus does not replicate in the cells.