A large number of pipe failures caused by wall thinning have been reported in nuclear power plants, some of which occur in a dual elbow or the vicinity of it. These pipe failures could be influenced by complex flow induced in the elbow. This study, therefore, aims at predicting the whole flow structure in the dual elbow as the first step by taking a secondary flow after the elbow by Ply measurement. A test section consists of two elbows that are 2-dimentionally connected with/without a straight pipe. They are made of acrylic resin. The diameter of thc elbow is 56mm and the curvature radius ratio is 1.5. Reynolds number in this experiment is 4x I It is confirmed that the flow structure in the dual elbow has specificity depending on the inlet flow condition to the elbow and that the secondary flow itself has swirling motion in a streamwise direction. The dual elbow seems to generate more complex and unstable flow field even when the flow field at the inlet of the elbow slightly changes from a fully developed flow. However, there is a strong possibility that putting a straight pipe between the two elbows makes it ease the occurrence of the complex flow field.