An immunoassay for selectively measuring human C-peptide as well as a kit therefor is disclosed. In the method, human C-peptide contained in a sample, a first anti-human C-peptide antibody, and a second anti-human C-peptide antibody which is immobilized on a solid support are reacted to form an immune complex among these three components. The formed immune complex is separated from the non-reacted antibodies and sample; and then the separated immune complex is quantified. The first antibody recognizes an epitope existing in the region from 1st to 16th amino acid residue from the N-terminal of the human C-peptide, and the second antibody recognizes an epitope existing in the region from 1st to 16th amino acid residue from the N-terminal of human C-peptide; with the proviso that the first and second antibodies do not recognize the same epitope so that they can simultaneously bind to said human C-peptide.