To make highly sensitively measurable the amount of carboxymethylation protein and average carboxymethylation rate in specimen solution by hydrolyzing liberated carboxymethylated(CM) protein into units of amino acid, and by measuring the amount of carboxymethylated amino acid.

Specimen solution such as blood, urine, and lymph is a solution containing an arbitrary amount of protein that can be measured, and may be a solution where carboxymethylated protein may go into the solution. First, the specimen solution is brought into contact with an antibody immobilization carrier, and the carboxymethylated protein in the specimen solution is connected to the antibody immobilization carrier by antigen antibody reaction. In this case, a batch method and column method are preferably used. Then, an insoluble carrier where the carboxymethylation protein is connected is separated from the specimen solution. After that, the carboxymethylation protein is liberated from the insoluble carrier, the liberated carboxymethylation protein is decomposed into an amino acid unit, and carboxymethylation amino acid is measured, thus determining the amount of existing carboxymethylated protein.