Flow structure and flow loss generation in a transonic axial compressor has been numerically investigated by using a large-scale detached eddy simulation (DES). The data mining techniques, which include a vortex identification based on the critical point theory and a limiting streamline visualization with the line integral convolution (LIC) method, were applied to the DES result in order to analyze the complicated flow field in compressor. The flow loss in unsteady flow field was evaluated by entropy production rate, and the loss mechanism and the loss amount of each flow phenomenon were investigated for the first rotor and the first stator. In the first rotor, a shock-induced separation is caused by the detached shock wave and the passage shock wave. On the hub side, a hub-corner separation occurs due to the secondary flow on the hub surface, and a hub-corner separation vortex is clearly formed. The flow loss is mainly caused by the blade boundary layer and wake, and the loss due to the shock wave is very small, only about 1 percent of the total loss amount in the first rotor. However, the shock/boundary layer interaction causes an additional loss in the blade boundary layer and the wake, which amount reaches to about 30 percent of the total. In the first stator, the hub-corner separation occurs on the suction side. Although only one hub-corner separation vortex is formed in the averaged flow field, the hub-corner separation vortex is generated in multiple pieces and those pieces interfere with each other in an instantaneous flow field. The hub-corner separation generates huge loss over a wide range, however, the loss generation around the hub-corner separation vortex is not so large, and the flow loss is mainly produced in the shear layer between the mainstream region and the separation region. The main factors of loss generation are the boundary layer, wake and hub-corner separation, which account for about 80 percent of the total loss amount in the first stator.