A method for purifying a complex, comprising: a binding step of causing a complex of a silicon oxide-binding tag and a substance to specifically bind to silicon oxide in a solution for binding, to which solution no salt is added; and a dissociating step of dissociating the complex from the silicon oxide, the dissociating step being a step of causing arginine to break binding between the silicon oxide-binding tag and the silicon oxide, wherein: the silicon oxide-binding tag consists of (i) a polypeptide having not more than 7 amino acids, wherein not less than 3/7 and less than 7/7 of all amino acids constituting the polypeptide are arginine, wherein the polypeptide consists of a polypeptide of (a) or (b): (a) a polypeptide having the amino acid sequence of SEQ ID NO: 3; and (b) a polypeptide having the amino acid sequence of SEQ ID NO: 3 in which one amino acid is deleted, substituted, or added, the polypeptide having a silicon oxide-binding capacity, or (ii) a polypeptide of SEQ ID NO: 5 or 7, wherein the substance is a protein. a binding step of causing a complex of a silicon oxide-binding tag and a substance to specifically bind to silicon oxide in a solution for binding, to which solution no salt is added; and a dissociating step of dissociating the complex from the silicon oxide, the dissociating step being a step of causing arginine to break binding between the silicon oxide-binding tag and the silicon oxide, wherein: the silicon oxide-binding tag consists of (i) a polypeptide having not more than 7 amino acids, wherein not less than 3/7 and less than 7/7 of all amino acids constituting the polypeptide are arginine, wherein the polypeptide consists of a polypeptide of (a) or (b): (a) a polypeptide having the amino acid sequence of SEQ ID NO: 3; and (b) a polypeptide having the amino acid sequence of SEQ ID NO: 3 in which one amino acid is deleted, substituted, or added, the polypeptide having a silicon oxide-binding capacity, or (ii) a polypeptide of SEQ ID NO: 5 or 7, wherein the substance is a protein. (i) a polypeptide having not more than 7 amino acids, wherein not less than 3/7 and less than 7/7 of all amino acids constituting the polypeptide are arginine, wherein the polypeptide consists of a polypeptide of (a) or (b): (a) a polypeptide having the amino acid sequence of SEQ ID NO: 3; and (b) a polypeptide having the amino acid sequence of SEQ ID NO: 3 in which one amino acid is deleted, substituted, or added, the polypeptide having a silicon oxide-binding capacity, or not less than 3/7 and less than 7/7 of all amino acids constituting the polypeptide are arginine, wherein the polypeptide consists of a polypeptide of (a) or (b): (a) a polypeptide having the amino acid sequence of SEQ ID NO: 3; and (b) a polypeptide having the amino acid sequence of SEQ ID NO: 3 in which one amino acid is deleted, substituted, or added, the polypeptide having a silicon oxide-binding capacity, or (a) a polypeptide having the amino acid sequence of SEQ ID NO: 3; and (b) a polypeptide having the amino acid sequence of SEQ ID NO: 3 in which one amino acid is deleted, substituted, or added, the polypeptide having a silicon oxide-binding capacity, or (ii) a polypeptide of SEQ ID NO: 5 or 7, wherein the substance is a protein.