A diagnostic reagent for hepatitis C virus infection obtained by sensitizing a solid phase with HCV antigen and a conjugated antigen prepared by chemical bonding of HCV antigen and a carrier protein, and a method of diagnosing hepatitis C virus infection, which comprises adding the diagnostic reagent for hepatitis C virus infection to a sample, and measuring the degree of agglutination of carrier particles as the solid phase. The diagnostic reagent and the method of diagnosis enable many samples to be measured with higher sensitivity and rapidity.