Target building is an 8-story steel encased reinforced concrete building which was constructed in 1998. In this structural health monitoring system, strong motion observation data is used and accelerometers were installed just after the completion of construction at 1st story, 2nd story, 5th story and 8th story. By use of system identification using ARX model, natural frequency, damping ratio and participation function are calculated, and concentrated equivalent story stiffness can be also determined by using Moore and Penrose generalized inverse matrix. From the identification results, natural frequency and concentrated equivalent story stiffness tend to decrease by the aging. Especially, just after the completion of construction and after a large earthquake, changes of natural frequency and concentrated equivalent story stiffness are very remarkable. From the point of amplitude dependence, natural frequency and concentrated equivalent story stiffness tend to change more by equivalent velocity of input energy than by peak ground acceleration. Analytical frame model is constructed from the structural design documents and concentrated equivalent story load-displacement relationships are obtained by carrying out push-over analysis. By the comparison between analytical and identified concentrated equivalent story stiffness, the structural conditions is estimated. From the identification results, a model using stick-slip elements is proposed. Natural frequency and story stiffness described by this model are consistent with identified results.