Abstract: There is a difficulty in workability of installing an optical fiber cable on a concrete structure, and when the optical fiber cable is installed on the surface of the concrete structure, the appearance of the concrete structure is spoilt by cutting the surface of the concrete structure or the like. Accordingly, there are provided a sheet-like strain sensor for confirming progress of damage of a concrete structure, in which one or a plurality of optical fiber cables are fixedly held between sheet-like bodies which are easy for the adhesive to permeate while one end/ends of the optical fiber cable/cables are pulled out; and a method for confirming progress of damage of a concrete structure by measuring strain by use of the one end of the optical fiber cable pulled out between the sheet-like bodies.