A method for producing a group of circular DNA molecules, the method comprising the following steps, wherein the method enables to select only single-molecule circular DNA that is not derived from multiple-molecule circular DNA from the group of circular DNA molecules produced by the method: 1) a step of binding an adapter for a first circularization to one end of each DNA molecule of interest, and binding to the other end of said DNA molecule of interest an adapter for a second circularization comprising an adapter (b) and an adapter, wherein the adapter binds to the DNA molecule via the adapter (b) side, such that the adapter in the adapter is located outside of the bond of the DNA molecule to the adapter (b), wherein the adapter comprises a cleavage site generating a cleavage end which enables non-specific binding between all of the cleavage ends of the adapter, the adapter (b) comprises two unique sequences different depending on each adapter (b), wherein the two unique sequences are identical in sequence and oriented in the same direction or reversely to each other; and between the two unique sequences, the adapter (b) also comprises a cleavage site generating a cleavage end which enables non-specific binding between all of the cleavage ends of the adapters (b), wherein the cleavage site generates a cleavage end that is not cleaved upon cleaving the cleavage site of the adapter and does not bind to the cleavage end of the adapter; 2) a first cleavage step of cleaving the DNA molecule obtained in step 1) at the cleavage site of the adapter; 3) a first circularization step of binding both ends of the DNA molecule obtained in step 2) to circularize the DNA molecule; 4) a step of eliminating an uncircularized linear DNA molecule in step 3); 5) a second cleavage step of cleaving the circular DNA molecule obtained in step 3) and step 4) at the cleavage site of the adapter (b); and 6) a second circularization step of binding both ends of the DNA molecule obtained in step 5) to circularize the DNA molecule, wherein the multiple-molecule circular DNA means circular DNA formed from multiple DNA molecules, and wherein the circular DNA molecule obtained in step 6) is determined to be single-molecule circular DNA that is not derived from multiple-molecule circular DNA, when two unique sequences contained in an adapter (b) portion are identical to each other as a result of determination of the sequence of the adapter (b) portion, and the circular DNA molecule obtained in step 6) is determined to be multiple-molecule circular DNA or single-molecule circular DNA derived from the multiple-molecule circular DNA, when the two unique sequences are different from each other. 1) a step of binding an adapter for a first circularization to one end of each DNA molecule of interest, and binding to the other end of said DNA molecule of interest an adapter for a second circularization comprising an adapter (b) and an adapter, wherein the adapter binds to the DNA molecule via the adapter (b) side, such that the adapter in the adapter is located outside of the bond of the DNA molecule to the adapter (b), wherein the adapter comprises a cleavage site generating a cleavage end which enables non-specific binding between all of the cleavage ends of the adapter, the adapter (b) comprises two unique sequences different depending on each adapter (b), wherein the two unique sequences are identical in sequence and oriented in the same direction or reversely to each other; and between the two unique sequences, the adapter (b) also comprises a cleavage site generating a cleavage end which enables non-specific binding between all of the cleavage ends of the adapters (b), wherein the cleavage site generates a cleavage end that is not cleaved upon cleaving the cleavage site of the adapter and does not bind to the cleavage end of the adapter; 2) a first cleavage step of cleaving the DNA molecule obtained in step 1) at the cleavage site of the adapter; 3) a first circularization step of binding both ends of the DNA molecule obtained in step 2) to circularize the DNA molecule; 4) a step of eliminating an uncircularized linear DNA molecule in step 3); 5) a second cleavage step of cleaving the circular DNA molecule obtained in step 3) and step 4) at the cleavage site of the adapter (b); and 6) a second circularization step of binding both ends of the DNA molecule obtained in step 5) to circularize the DNA molecule, wherein the multiple-molecule circular DNA means circular DNA formed from multiple DNA molecules, and wherein the circular DNA molecule obtained in step 6) is determined to be single-molecule circular DNA that is not derived from multiple-molecule circular DNA, when two unique sequences contained in an adapter (b) portion are identical to each other as a result of determination of the sequence of the adapter (b) portion, and the circular DNA molecule obtained in step 6) is determined to be multiple-molecule circular DNA or single-molecule circular DNA derived from the multiple-molecule circular DNA, when the two unique sequences are different from each other.