A method is proposed to analyze the vibration of jack-up rig using a model made by replacing its legs and platform with equivalent beams and rigid body, respectively, and connecting them by equivalent springs. The transfer matrix method is applied to analyze the natural vibration of the above mentioned model. The accuracy of this method is studied by comparing the results calculated by this method with those calculated by the detailed method assuming legs as space frames and platform as plane grid structure and also with the measured data obtained by the full scale measurement, and it has become clear that the method in this paper gives practically sufficient results in shorter computer time compared with the detailed method. Thus, this method seems to be very useful not only for the analysis of natural frequency and mode but also for the analysis of forced vibration by waves.