To provide a distortion-measuring method and device which do not require use of a large number of gauges, or special knowledge, techniques, or expensive apparatus, and moreover provide a method and device capable of detecting internal damages, even if damaged parts and broken points are at a location in a base material and detecting internal damages, without embedding an optical fiber or a piezoelectric material in the material, and composite materials to be used for the same.

A composite reinforcing fiber (10), with a conductive core material (12) and a coating layer (13), with which the core material (12) is coated and which has an electrical resistance substantially larger than the core material (12), is embedded in works (W and WA) of resin, metal, ceramics, and a composite material, and both ends (10E and 10E) of the core material (12) are connected to a resistance-variable electrical resistance measuring device (16) and the distortion-measuring device (18). Variations (δR) in the electrical resistance which occur in the composite reinforcing fiber (10) are converted into distortions, to measure the distortion in the works (W and WA).