This article describes how two seemingly contradictory concepts of social stratification in contemporary Japan— "fluidization" and "stabilization" —can be understood and explained in a coherent manner. A classic example of "fluidization" is an increase in non-regular workers, while that of "stabilization" is an increase in the rigidity of the intergenerational social mobility of a particular stratum. We hypothesize that fluidization has not necessarily occurred in all strata; some strata are still under the umbrella of protective institutions, while others are being influenced by the rapidly increasing fluidity. By examining empirical findings of the 2005 Social Stratification and Social Mobility Project, we argue that this hypothesis is generally supported. In the concluding section, we consider the implications of the coexistence of stability and fluidization in the study of social stratification.