A standby safety system in nuclear power plant is periodically tested to detect a failure in the system. The unavailability due to periodical testing dominently contributes to the system unavailability. Therefore, determination of test interval is an important problem in the aspect of system availability. In current state of system availability modeling, the system has been assumed to return to a good-as-new state (i.e., turn to a perfect component) after test and repair. But we can not obtain the practical result of optimal test interval by this modeling. This paper deals with a constant unavailability which consists of the unavailability due to human error and the inherent unavailability for the modeling of optimal test interval. Inherent unavailability is summation of the inherent unavailability on demand and the unavailability of a support system. And it is defined as the unavailability, which is caused by an inherent undetectability. Consideration of the constant unavailability makes it possible to determine the rational optimal test interval of the redundant component system. The results show that the optimal test interval in redundant component system dominently depends on the constant unavailability. As a result, the developed modeling makes it possible to determine the more exact and practical test interval in redundant component system and gives the more improved prediction of system availability.