This paper presents an improved side-channel attack based on multivariate regression analysis. The proposed method effectively refines measured side-channel traces using profiled information. Our method is advantageous to conventional profiling attacks as it is robust against attack configurations; it requires less traces while profiling and is less sensitive to the number of interesting points. We demonstrate the effectiveness of our method through concrete experiments in comparison with conventional methods. In the experiments, the proposed method is examined under various configurations to investigate its performance.