Cracks are inherent to concrete by its nature. The various size and shape of cracks induce deterioration of reinforced concrete structures including nuclear power plants. The wider and deeper the crack is, the concrete structures are more vulnerable to carbonization. Thus, it is essential to develop a reliable measurement technique of cracks inside concrete. In this study, an ultrasonic test method is applied to the crack measurements. The results can be used for evaluation of existing reinforced concrete structures.