The present invention addresses the problem of providing a novel method for suppressing the nonspecific adsorption of proteins. Knowledge enabling the abovementioned problem to be solved was obtained, leading to the completion of the present invention, by treating a solid phase in a solid-phase treatment solution having a phosphorylcholine group-containing polymer as the main component, treating a sample in a sample treatment solution having a polymer comprising a phosphorylcholine group, a hydroxyl group and a hydrophobic group as the main component, and then adding the treated sample to the treated solid phase.