Bulging is a coupled vibration between tank wall and content fluid. This phenomena is induced by seismic ground motion. In Japan, many cases of water storage tanks due to bulging have been reported with an intensity index over 6. Seismic design against bulging is introduced just at 2022, which will activate various kind of engineer’s interest on it. That is e.g. possibility of phenomena occurrence for existing tanks. In this study, microtremor measurement and natural frequency analysis are conducted to estimate the natural frequencies of the real scale SUS panel tanks, which are considered to have natural frequencies below 10 Hz, the frequency at which bulging damage is expected to occur. As a result, the calculated natural frequency of bulging is consistent with that of the actual tank, indicating the possibility of bulging occurring in the tank. The microtremor measurement method is shown to be particularly effective for SUS panel tanks, which are considered to have natural frequencies below 10 Hz.