Provided is a novel method of suppressing non-specific protein adsorption. The above-mentioned object can be achieved by treating a solid phase with a solid phase-treating liquid containing a phosphorylcholine group-containing polymer as a main component, and treating a specimen with a specimen-treating liquid containing, as a main component, a polymer having a phosphorylcholine group, a hydroxy group, and a hydrophobic group, followed by addition of the treated specimen to the treated solid phase.