There have been various papers about service quality. This article is one of them. This is about the measurement of service quality in the competitive structure between department and discount store. In this paper, we tried to measure the service quality and overall satisfaction by using 6 sigma, degree of combination and top2box which is a little bit different methodology from traditional ones. The data were collected by the internet survey from 1428 and 1605 department and discount store customers respectively. The result shows the different patterns in the each retail industry. Also, there is a significant difference in terms of sigma level in the each retail industry. Finally, we showed the summarized result as the 6 Sigma Portfolio Matrix.