The present invention is for measuring the diameter of a steel reinforcement rod in a concrete. Therein, a comparing rod is deposed outside of the construction and is corresponding to the steel rod to make the magnification rates of the steel rod and the comparing rod be the same. Then, the comparing rod and the steel rod are irradiated by a radioactive-ray source. And, then, a display is provided to show images of the rods to evaluate the size and the defect of the steel rod by the size and the shape of the comparing rod, so that the examination accuracy can be greatly improved and the examination result can be a reference for evaluating the construction safety.