Seismic responses of steel buildings with curtain walls having bilinear hysteresis restoring force characteristics are analysed. A modal method of analysis is applied to. the buildings idealized as multi-degree-of-freedom systems to obtain equivalent (non-linear) one-mass systems. An equivalent linealization method is also used to solve the equation of motion of the one-mass systems, and is shown to be practical enough to calculate the approximate responses of the buildings of ordinary sizes. An existing building is calculated by the method, and it is ascertained that accuracy is sufficient and calulation time is much decreased.