We investigated the impact of the interface roughness in W-CoFeB-MgO-CoFeB-W structures on MgO reliability using the atomic force microscopy images of a MgO underlayer surface and I-V measurements. As a result, the electric current in the MgO film increased as the MgO underlayer surface roughness increased. We assumed that this phenomenon is caused by the local electric field concentration, and verified it by comparing the two electric field concentration values simulated from the AFM images and calculated from the I-V measurements. These two electric field concentrations calculated from the different results have a strong correlation to each other. It is concluded that the current of the MgO film is increased by the large roughness between MgO and the electrodes. This causes the large current variation in memory cells in magnetic random access memory (MRAM), then it leads to a small margin in memory operation.