The ultimate subject of the present study is concerned with the design of a nonlinear control law with integral compensation for the output voltage of boost converters through the use of their discretized bilinear model. In this first part of the study, we begin with a discretized model of boost converters obtained by rigorously taking account of the switching action in the converters. Since the control input, the duty cycle, is involved in this model in a highly nonlinear fashion,however, designing a discrete-time control law with this model is rather unrealistic. To circumvent this difficulty, we approximate the discretized model and introduce a simplified model, a discretized bilinear model, and discuss its favorable property. We further give a method for experimentally determining the discretized bilinear model through system identification. With an experiment on identifying an actual boost converter, we then discuss the effectiveness of the discretized bilinear model and its identification method. The resulting model will be used for discrete-time control law design to verify effectiveness of the overall scheme of the present study in an accompanying paper.