This paper sets out to investigate the potentials of using pupil diameter measure as a contactless biofeedback method. The investigation was performed on how the interdependent and competing activation of the autonomic nervous system is reflected in the pupil diameter and how it affects the performance on decision-making task in serious games. The on-line biofeedback based on physiological measurements of arousal was integrated into the serious game set in the financial context. The pupil diameter was validated against the heart rate data measuring arousal, where the effects of such arousal were investigated. It was found that the physiological arousal was observable on both the heart and pupil data. Furthermore, the participants with lower arousal took less time to reach their decisions, and those decisions were more successful, in comparison to the participants with higher arousal. Moreover, such participants were able to get a higher total score and finish the game. This study validated the potential usage of pupil diameter as an unobtrusive measure of biofeedback, which would be beneficial for the investigation of arousal on human decision-making inside of serious games.