As a substance which pharmacologically regulates the function of a cell surface functional molecule, a substance which has specificity and an activity or efficacy equal or superior to an antibody and does not require an advanced production technique and facility for application thereof to a pharmaceutical product has been demanded. The invention relates to a multimer of an extracellular domain of a cell surface functional molecule, particularly a tetramer of an extracellular domain of PD-1 or PD-L1. Further, the invention relates to an application of such a tetramer as a preventive and/or therapeutic agent for cancer, cancer metastasis, immunodeficiency, an infectious disease or the like and an application of PD-1 or PD-L1 as a testing or diagnostic agent or a research agent for such a disease.