A dual vibration observation system (DVOSYS) is proposed for structural health monitoring study, which can amass both strong earthquake response and continual long-term ambient vibration. The advantage of the DVOSYS is to obtain complete sets of an earthquake record and the ambient vibration records before and after the earthquake, and these records are useful to establish vibration-based damage detection. The authors have been monitoring a baseisolated building since August 16, 2004. Recent results show that the natural frequencies have diurnal fluctuations where the first and second natural frequencies are varying mutually independent, and that a far field earthquake induces rigid-body mode of the base-isolated building with the surrounding ground, which are caused by long period ground motion owing to the deep underground structure of Kanto Basin. These results can be introduced only by the long-term vibration monitoring function of the DVOSYS.