The bolt joints on steel structures are exposed to the possibility of damage, and thus, require intensive care. Usually, periodic inspections are conducted at the cost of time and money. However, it is very difficult to check so many bolts carefully. The purpose of this study is to propose a system that can more efficiently monitor the tightness/looseness of these bolts. The proposed bolt fastening monitoring system is comprised of sensors that are attached to nuts and a data receiving terminal, which gathers information. The reed switch consists of two thin, metallic contacts enveloped in a glass tube and is an electrical switching sensor that is triggered ON or OFF by changes in the surrounding magnetic field. The verification tests showed that bolt loosening can be effectively detected, proving the applicability of this system to the maintenance of the bolt joints of steel structures. The newly developed sensor system is expected to solve conventional sensor problems by enabling measurement of structural members which was not previously possible, thus providing a basis for a new technology in the construction industry by applying IT to construction technology.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.