The purpose of this study was to examine the mediating effects of emotional dysregulation on the relationship between behavioral inhibition system (BIS), behavioral activation system (BAS), and anxiety and aggression. BAS basically consisted of three factors. However, in our study, the BAS score was composed of two factors, drive and fun-seeking, which are expected to increase the risk of aggression. According to the joint subsystem hypothesis, BIS and BAS can mitigate each other’s effects on psychological functions. Thus, we analyzed both the main effects and interaction effects of BIS and BAS. Participants were 300 adults (195 males and 105 females) who responded to the self-reporting questionnaires assessing BIS/BAS, difficulties in emotion regulation, state-trait anxiety, and aggression. The result of path analysis of anxiety indicated that the BIS positively predicted anxiety, and emotional dysregulation partially mediated their relationship. In contrast, the total effects of BAS on anxiety were non-significant. The interaction effects of BIS and BAS were significant, which meant that the BAS mitigated the relationship between BIS and anxiety. The result of path analysis of aggression indicated that the BAS positively predicted aggression, and emotional dysregulation partially mediated their relationship. In contrast, the indirect effects of the BAS on aggression through emotional dysregulation were positively significant, but the direct effects and total effects were non-significant. The interaction effects of BIS and BAS were also not significant which meant that the BIS did not mitigate the relationship between BAS and aggression. These findings suggest that we should consider both BIS and BAS in order to explain anxiety and aggression and importance of emotional dysregulation as a transdiagnostic factor.