It is very important to predict natural frequencies and modes of a structure vibrating with fluid in the design of turbomachines such as water turbines and gas turbines. As for reversible water turbines, it is necessary to analyze complex and large-scale problems such as coupled vibration of a runner and an upper cover which sandwich the water in a narrow space. Using the conventional method it is quite difficult to solve such problems because of the memory limit of computers due to the increase in the number of degrees of freedom of the system. This paper proposes an analysis method for prediction of natural frequencies and modes, which enables deduction of the size of the matrices by introducing the modal method for both structure and fluid. The objective of this method is to obtain converged added mass for necessary modes by iterative modification of this added mass in modal coordinates until the orthogonality is satisfied. The validity and the usefulness of this method are confirmed by comparison with measured data.