Disclosed is a method by which the adsorption of cystatin C to a container can be inhibited in a simple manner to improve the accuracy of the measurement of cystatin C. Provided are: a cystatin C adsorption inhibitor comprising a non-ionic surfactant; a cystatin C measurement reagent comprising the adsorption inhibitor; and a cystatin C measurement kit. Also provided is a method of inhibiting the adsorption of cystatin C, the method comprising bringing a cystatin C-containing sample into contact with a measurement instrument in the presence of a non-ionic surfactant. The aforementioned non-ionic surfactant is preferably a polyoxyethylene-type surfactant. Alternatively, the aforementioned non-ionic surfactant has preferably a phenoxy structure, more preferably a benzylphenoxy structure.