Sensing part of this system shown in Figure 4 is a circuit in which electrically conductive material connects with copper wire part of the RFID tag. This sensing part is attached to detection structure part, and communication is carried out between transmitter and RFID tag. If the electrically conductive material is cut off and the communication between RFID and transmitter is not possible, we can know the level of crack at the point electrically conductive material is applied. That is, the crack width is estimated by the ability of the electrically conductive material to conduct electricity [14].