When an interlocking device is improved, a large number of signal cables have to be re-laid and wiring connections must be confirmed. There are possibilities of wiring errors in the improvement of the interlocking device. A signal control system using an optical LAN is developed for the purpose of reduction of signal cables and wiring work. This system controls signal devices by transmitting digital information through an optical network, and it has a method of system configuration to make the system more flexible and secure. To introduce this system effectively, an application of assurance technologies is necessary because this system coexists with the existent interlocking device over the period of construction and operation. In this paper, we introduce this system and describe assurance technologies used for this system.