Provided is a method for amplifying cyclic DNA, particularly long-chain cyclic DNA, simply and exponentially in a cell-free system. More specifically, a cyclic DNA amplification method is provided, which comprises allowing to react a reaction mixture produced by mixing cyclic DNA having an origin sequence of chromosomal replication (an origin of chromosome (oriC)) with a reaction solution, wherein the reaction solution contains enzymes (1) to (3) as mentioned below, a buffer solution, NTP, dNTP, a magnesium ion source and an alkali metal ion source : (1) a first group enzyme which can catalyze the replication of cyclic DNA; (2) a second group enzyme which can catalyze an Okazaki fragment connection reaction to synthesize two sister cyclic DNA molecules capable of forming a catenane; and (3) a third group enzyme which can catalyze a reaction of separating two sister cyclic DNA molecules from each other.