Method of measuring target substance, and reagent therefor

A reagent for measuring a target substance in a specimen,

• wherein the target substance is a protein forming a complex that is hexameric or higher,
• wherein the reagent includes a first reagent including a buffering agent, and a second reagent including a particle on which a ligand that specifically binds to the target substance is immobilized, and
• wherein, when a first mixing step of obtaining a first mixed solution, in which the specimen containing 1,000 ng/ml of the target substance and the first reagent are mixed at a volume ratio “specimen:first reagent” of 15:60, and a second mixing step of obtaining a second mixed solution, in which the first mixed solution and the second reagent are mixed at a volume ratio “first mixed solution:second reagent” of 75:30, after 280 seconds or more have elapsed from the mixing in the first mixing step are performed, an amount of change in absorbance of the second mixed solution within 50 seconds from the mixing in the second mixing step is 0.15 or more.