Free vibration is analyzed for unsymmetrical joined plate or shell structures with a closed member by means of the transfer matrix method. For this purpose, the continuity and equilibrium relations for the displacements and forces at the joints are written with use of the joint matrices. The connection matrix between the closed member and other members at the joints is derived by introducing the structure matrix of the closed member, and the entire structure matrix is obtained by the product of the connection matrix of the closed member and the transfer matrices of other members. This method is applied to a plate structure with an unsymmetrically sited duct and a box‐type structure with an aslant interior plate, and the natural frequencies are calculated numerically together with the mode shapes of vibration giving the results.