Introduction: Social anxiety disorder (SAD) symptoms are maintained by cognitive biases, which are overestimations of the severity and likelihood of negative social events (cost/probability biases), and by sensitivity to rewards and punishments that are determined according to behavioral inhibition/behavioral activation systems (BIS/BAS). Cost/probability biases might activate the behavioral immune system and exacerbate the avoidance of social events. Earlier studies have proposed that low BIS or high BAS decrease SAD symptoms; BIS/BAS may even change the effects of cognitive biases on SAD symptoms. Hence, the current study investigates the interaction effects of BIS/BAS and cost/probability biases on SAD symptoms. Method: Seventy-six Japanese undergraduate students completed the Japanese version of the Liebowitz Social Anxiety Scale (LSAS), which comprises Fear and Avoidance subscales, the BIS/BAS Scale, and the Social Cost Probability Scale. Results: A multiple regression analysis was performed to examine whether cost/probability biases, BIS/BAS, and their interactions affected SAD symptoms; following this, the main effects of cost bias and BIS were determined for LSAS-Fear (β = 0.64, p < 0.001; β = 0.33, p < 0.01) and LSAS-Avoidance (β = 0.49, p < 0.001; β = 0.35, p < 0.01). The interaction effect between cost bias and BAS was significant for LSAS-Avoidance (β = -0.32, p < 0.05). Simple slope analysis showed that the slope of cost bias was significant for low-BAS individuals (β = 0.77, p < 0.001) but not for high-BAS individuals (β = -0.21, n.s.). The interaction effect between probability bias and BAS was significant for LSAS-Avoidance (β = 0.40, p < 0.01) as well. Further, simple slope analysis revealed that the slope of probability bias was significant for low-BAS individuals (β = -0.53, p < 0.05) but not for high-BAS individuals (β = 0.17, n.s.). Discussion: The study found interesting results with respect to the avoidance of social events. Low-BAS individuals with high cost or low probability biases regarding social events may have a tendency to avoid social events. In contrast, if high-BAS individuals overestimate the cost of social events or underestimate the probability of social events, their anticipation of rewards might prevent them from avoiding social events.