An object of the present invention is to detect a human CXCL1 protein with high sensitivity. An immunoassay method is provided for a human CXCL1 protein, by which human CXCL1 or a fragment thereof in a sample is measured using two or more types of anti-human CXCL1 monoclonal antibodies or fragments thereof, wherein: each of the two or more types of anti-human CXCL1 monoclonal antibodies or fragments thereof specifically recognizes any one of sequence regions of the amino acid sequences shown in SEQ ID NOS: 1-3, which are partial sequences of the amino acid sequence composing a human CXCL1 protein; and the two or more types of anti-human CXCL1 monoclonal antibodies or fragments thereof specifically recognize sequence regions that differ from each other. Monoclonal antibodies or fragments thereof are provided, each of which specifically recognizes any one sequence region of the amino acid sequences shown in SEQ ID NOS: 1-3 and has a new amino acid sequence.