In this study, the effects of a cognitive task on the following unpleasant emotions were examined. For indices of the unpleasant emotions, we asked participants subjective reports of their emotions and measured physiological responses such as heart rate (HR) and skin conductance level (SCL). The participants were randomly divided into a mental arithmetic group and a control group. The former group pursued the mental arithmetic task and the latter group stayed calm before two trials of an emotional task, in which the participants were asked to view and evaluate affectively unpleasant and neutral pictures. As results, the physiological responses during the emotional task were inhibited and the subjective negative affect was lower in the mental arithmetic group, compared to the control group. Moreover, these effects were observed through the second emotional task, which was done for checking the rebound effects. The inhibition effect of the physiological responses was maintained until the recovery session. In conclusion, it was suggested that the cognitive activity such as mental arithmetic affects the following emotional reactivity, including subjective unpleasantness and the physiological responses (HR and SCL). We discussed the roles of cognitive task in regulation of unpleasant emotion, especially focused on the duration of its effect.