In this paper, we examined the effect of deliberative democracy on the polarization of opinion. Through three case analyses, we find that deliberative democracy has two major components - provision of information and group discussion. Polarization of opinion can be explained by several theories: social identity theory, social comparison, and confirmation bias. We constructed a deliberative democracy model that reflected these mechanisms of polarization. We found that deliberative democracy actually decreased polarization of opinion when group discussion was a strong factor. Our study provides guidelines for an institutional design incorporating deliberative democracy, with emphasis on the composition of unbiased group discussions.