The procedure for identifying linear time-varying systems with dead times by using the wavelet analysis has been already proposed by the authors. This paper reports an application of the identification procedure to actual control systems of water-tube drum boiler in a thermal power plant. The control systems are required to supply steadily high-quality steam against disturbances due to unexpected changes in load. Therefore, it is necessary to make a model which can take the disturbances into consideration for control system design. The identification procedure is composed of the estimation of the stationary-state time range and that of the process transfer functions in the stationary and non-stationary states. The identified model sets are evaluated by the prediction errors in the process outputs, drum-pressure and drum-level, in comparison with an ARX model.