Regardless of the type of specimen, such as a serum or heparinized plasma containing different anticoagulants, which are widely used in general, when a substance to be measured (for example, sIL-2R) in a biological sample is immunologically detected, a measuring method and a kit capable of stably obtaining with high accuracy, unaffected by interfering substances in the specimen, are provided. An immunocomplex between the substance to be measured and an antibody that specifically binds to the substance to be measured is formed in the presence of a sulfated polysaccharide. The kit comprises an antibody that specifically binds to the substance to be measured, and a buffer containing a sulfated polysaccharide.