Maintenance technologies for the reactor system have been developed by using the High Temperature engineering Test Reactor (HTTR). One of the important purposes of development is to accumulate the experiences and data to satisfy the availability of operation up to 90% by shortening the duration of the periodical maintenance for future high-temperature gas-cooled reactors (HTGRs) by shifting from time-based maintenance to condition-based maintenance. The technical issue of the maintenance of the in-core neutron detector, wide range monitor (WRM), is to predict the malfunction caused by cable disconnection to plan the replacement schedule. This is because it is difficult to observe directly inside the WRM in detail. The electrical inspection method was proposed to detect and predict the cable disconnection of the WRM by remote monitoring from outside the reactor using the time-domain reflectometry (TDR). The disconnection position, which was specified by the electrical method, was identified by nondestructive and destructive inspection. The accumulated data are expected to be contributed for advanced maintenance of future HTGRs.