In this paper, a rotary-type unstable inverted pendulum system is modeled by using a parameterdependent linear-time-invariant (PD-LTI) system. The pendulum systems operating at upright and downright positions are modeled by a PD-LTI system with a scheduling parameter that is fixed in each operation. The unknown parameters are estimated by taking a grey-box modeling approach to the pendulum system operating at the downright position. It is also considered how to choose the model structure by taking numerical condition into account.