Sosokan, a building at theYagami Campus of Keio University, has a semi-active control system that include 8 semi-active dampers and 24 passive dampers installed in the base-isolation layer. The parameters of the semi-active controller were designed using the initially assumed design values of the building. However, these parameters may not be identical to its present actural condition and can be updated on basis of observation records. This paper aims to improve the performance of the controller through identification of the building properties by updating the control parameters on basis of the records of seismic responses during the main shock and aftershocks of the 2011 Great East Japan Earthquake. In the identification, errors between the time history responses of the main shock records and simulation results are minimized by an optimization scheme. Thereafter, structural properties of the updated building model is applied to improve the performance of the semi-active controller.