The three-dimensional flow structure induced by normal shock wave/turbulent boundary-layer interaction in a transonic diffuser is investigated by the numerical simulation. This simulation is compared quantitatively with the results of pressure measurement, and represents good agreement. The computational flow has the complicated shape of the boundary layers and three-dimensional vortices generated at the foot of the bifurcated shock wave. The vortices are evidenced experimentally in the oil-flow surface visualization. The computational result reveals that the complicated wave configuration is formed at the diffuser corner. The simple flow model is constructed by considering this wave configuration. This model can explain qualitatively very well the three-dimensional flow characteristics.