This paper reports a simple damage detection method applied to a building under an earthquake excitation. The structure is modeled as a single-input-single-output ARX model, which means only two measurements in each direction. The model parameters are linearly identified by the forgetting factor recursive least-squares algorithm. When the structure is damaged, its natural frequencies decrease and the corresponding damping ratios increase. The identification can understand the time of damage occurrence directly and the damage level indirectly. The applicability is verified by shaking table tests of a full-scale 4-story steel building at the E-Defense.