The cracks detected by in-service inspection are not always removed when they are not hazardous according to fitness-for-service evaluations. In such cases, it is important to monitor the growth of these cracks in order to assess the validity of their integrity assessment. However, due to the limitation of the accuracy of size determination by ultrasonic testing, it is difficult to know how much the cracks have grown since the previous measurement. In this study, a method for crack growth monitoring during plant operation is proposed. When a pipe is deformed elastically due to internal pressure, the strain at its external surface increases. By measuring the change in strain for the outside of the cracked pipe continuously, it is possible to determine how much the crack size changes. Elastic finite element analyses were performed for cracked pipes under internal pressure. From the analyses under various crack sizes, the accuracy and resolution of the proposed monitoring method were evaluated. It was revealed that the method could detect crack growth of less than several hundred micrometers.