This thesis considers an ordering policy for a system composed of a single equipment and a sensor, where the operational condition of the equipment is monitored by the attached sensor and two kinds of procurement lead times affect the system costs. Furthermore, both the equipment and the sensor are subject to random failure. Here we use the cost effectiveness as a criterion for optimality and lead to an optimum ordering policy maximizing it. Especially, we show under certain conditions that there exists a finite and unique optimum ordering policy maximizing it. Numerical examples are presented for illustration.