A method of constructing a DNA concatemer to be used for microbial cell transformation, the DNA concatemer comprising more than one DNA assembly unit, each of the more than one DNA assembly unit comprising a DNA vector harboring an origin of replication effective in a host microorganism and a DNA assembly, comprising: a step of preparing a DNA unit fragment composition in a solution, the step including; a step of preparing solutions containing multiple kinds of DNA unit fragments, each solution containing one of the multiple kinds of DNA unit fragments, each DNA unit fragment being attached to a corresponding auxiliary sequence; and a step of, after preparing each solution, measuring the concentration of each kind of DNA unit fragment with the corresponding auxiliary sequence attached thereto in each of the solutions, and then based on the measurement result, taking a portion from each of the solutions so that the number of moles of DNA unit fragment in one portion is close to the number of moles of DNA unit fragment in another portion; a step of preparing the DNA vector; a step of removing with a restriction enzyme a corresponding auxiliary sequence from each DNA unit fragment with the corresponding auxiliary sequence attached thereto contained in the solution after preparation; and a step of, after the removal step, joining the DNA vector and each of the DNA unit fragment together, wherein each of the DNA vector and the DNA unit fragment is structurally capable of being joined repeatedly while maintaining a certain order, and each DNA assembly comprises of a DNA molecule in which the DNA unit fragment is joined to one another; and a step of, based on a relation between the yield of a DNA fragment comprising a target number of DNA unit fragments joined together and a coefficient of variation for the concentration of this DNA fragment, the yield being equal to the product of the number of DNA unit fragments per assembly unit and the number of the assembly unit, adjusting a coefficient of variation for the concentrations of the DNA vector and each DNA unit fragment in the joining step. a step of preparing a DNA unit fragment composition in a solution, the step including; a step of preparing solutions containing multiple kinds of DNA unit fragments, each solution containing one of the multiple kinds of DNA unit fragments, each DNA unit fragment being attached to a corresponding auxiliary sequence; and a step of, after preparing each solution, measuring the concentration of each kind of DNA unit fragment with the corresponding auxiliary sequence attached thereto in each of the solutions, and then based on the measurement result, taking a portion from each of the solutions so that the number of moles of DNA unit fragment in one portion is close to the number of moles of DNA unit fragment in another portion; a step of preparing solutions containing multiple kinds of DNA unit fragments, each solution containing one of the multiple kinds of DNA unit fragments, each DNA unit fragment being attached to a corresponding auxiliary sequence; and a step of, after preparing each solution, measuring the concentration of each kind of DNA unit fragment with the corresponding auxiliary sequence attached thereto in each of the solutions, and then based on the measurement result, taking a portion from each of the solutions so that the number of moles of DNA unit fragment in one portion is close to the number of moles of DNA unit fragment in another portion; a step of preparing the DNA vector; a step of removing with a restriction enzyme a corresponding auxiliary sequence from each DNA unit fragment with the corresponding auxiliary sequence attached thereto contained in the solution after preparation; and a step of, after the removal step, joining the DNA vector and each of the DNA unit fragment together, wherein each of the DNA vector and the DNA unit fragment is structurally capable of being joined repeatedly while maintaining a certain order, and each DNA assembly comprises of a DNA molecule in which the DNA unit fragment is joined to one another; and a step of, based on a relation between the yield of a DNA fragment comprising a target number of DNA unit fragments joined together and a coefficient of variation for the concentration of this DNA fragment, the yield being equal to the product of the number of DNA unit fragments per assembly unit and the number of the assembly unit, adjusting a coefficient of variation for the concentrations of the DNA vector and each DNA unit fragment in the joining step.