This sensor is used for easy and accurate detection of cracks developed on the surface of concrete structures due to deterioration. The sensors are previously bonded onto a concrete structure and the wirings are finished. At the time of regular check, the light source is connected to one optical fiber cable connector after another to perform visual observation of multiple points in order. No power supply is needed on the site. The light source itself is very small and light, and the checking job can be securely performed. Detection accuracy of crack widths can be selected from 0.05 / 0.1 / 0.2 / 0.3 mm, allowing use for detection of cracks at the initial stage or repaired cracks. In addition to ability to connected several double-end KZCB units end-to-end, use of crack sensor adaptor KZCBF-11 allows a data logger to automatically detect cracks.