In general, a crack scale is used for measuring width of a surface crack in concrete structures. However, it seems infeasible from the standpoint of promotion and optimization of the method of a non-destructive testing for practical use. The purpose of this study is to develop a mobile crack measuring instrument and to evaluate the fundamental characteristics when use on various construction materials especially in measuring mainly width of a crack in concrete. As a result, the followings as the extent of measurement to be applied depending on the shape of a crack, an effect by characteristics of building surface materials, an effect of finishing colors, an effect of the roughness of the surface and an effect of skills of the measuring person became clear as pertinent in practice
