[en] Pipelines in power plants, nuclear facilities and chemical industries are often affected by corrosion effects. It is very important in detect such internal flaws of pipelines because they sometimes bring about serious problems. Conventional NDT methods have been taken relatively much time, money, and manpower because of performing as the method of contact with objects to be inspected. Digital shearography is a laser-based optical method which allows full-field observation of surface displacement derivatives. This method has many advantages in practical use, such as low sensitivity to environmental noise, simple optical configuration and real time measurement. In this paper, the experiment was performed with some pressure vessels which has different internal cracks. We measured internal crack length of the pressure vessels at a real time and compared with FEM simulation results.