[en] Seismic analysis of JT-60, a tokamak machine under construction in JAERI, was carried out in order to confirm machine integrity in earthquakes in connection with any aseismic design changes. Described are an analysis method newly developed for seismic analysis of JT-60 and the results of eigenvalue analysis of the machine. For analysis, the machine model comprises ''simple models'' of respective components constituting the machine. Each simple model is based on the engineering criterion that it should have as similar dynamic characteristics to those of a ''detailed model'', which is faithful to the real component structure and further as few nodal points as possible. So, analysis of the machine model gives information of not only overall behavior of the machine but also local vibration of each component with few nodal points. The analysis method is thus useful especially for large complex structures such as JT-60. The method was applied to JT-60; the machine model with about 800 nodal points is constructed of simple models of respective components having eigenmodes and frequencies similar to those of the detailed models up to the 10th mode. Eigenvalue analysis of the machine model showed that JT-60 has the 1st natural frequency of 4.5 Hz and distinguished horizontal vibration mode of the vacuum vessel (40⁰ sector); which indicates the necessity of design change of the support structure of the vacuum vessel. (author)