An assembling method for a multi-core cable having a plurality of electrical insulated wires is designed to connect one-end-portions of the electrical insulated wires to electrode patterns, respectively, of one circuit board, correspondingly connect other-end-portions of the electrical insulated wires to electrode patterns, respectively, of the other circuit board, compute intersection coefficients on one end side and the other of the cable, and iterate interchanging connecting destinations for the one-end-portions of the electrical insulated wires, correspondingly interchanging connecting destinations for the other-end-portions of the electrical insulated wires, and computing the intersection coefficients on the one end side and the other of the cable. The connecting destinations for the electrical insulated wires to the electrode patterns are determined in such a manner that a maximum intersection coefficient denoting either larger one of the respective intersection coefficients of the one end side and the other of the cable is made small.