Democracy and constitutionalism are usually regarded as indispensable elements of modern constitutions. Some constitutional scholars and political scientists, however, argue that constitutionalism and democracy are mutually incompatible, and that one or the other must be preferred. Particularly in Japan, both the constitutionalist and the most influential of the democratic positions presuppose that democracy is nothing but majority rule. Thus these scholars observe that in a parliamentary cabinet system a political opinion of the Prime Minister, who is the head both of the Cabinet and of the ruling (majority) party, clashes with jurists' or lawyers' ideas or interpretations regarding that country's constitution, thus showing that constitutionalism and democracy are incompatible. On the other hand, the layperson unfamiliar with political sciences or constitutional studies takes for granted that democracy and constitutionalism are compatible with each other; but there are few powerful arguments why democracy and constitutionalism are compatible with each other. I shall attempt to rectify the omission in this paper. Above all, despite the persistent idea of democracy as majority rule, majority rule is not a characteristic of democracy, as Popper has argued. Popper's theory of democracy, therefore, may provide a solution allowing for both democracy and constitutionalism as compatible elements of a constitution.