Meditation is a specific consciousness state in which deep relaxation and increased internalized attention coexist. Autonomic nervous activity during meditation was reported to be characterized by decreased sympathetic activity and increased parasympathetic activity. And concerning EEG power, slow alpha power and fast theta power increased during meditation. On the other hand, mental arithmetic (MA) tasks have been reported to induce an increase in blood pressure and heart rate with enhanced sympathetic activity. However, changes in autonomic nervous activity and EEG power during MA tasks and their mutual relationship have not been adequately studied. In the present study, we quantitatively analyzed the changes in autonomic nervous activity using heart rate variability (HRV) and EEG power during MA task in 30 healthy volunteers. MA task induced significant increases in normalized LF, LF/HF ratio (as a sympathetic index), and a decrease in normalized HF (as a parasympathetic index). On the other hand, significant decrease in EEG power (slow theta: 4-6 Hz and fast alpha: 10-13 Hz in the posterior region and fast theta: 6-8 Hz and slow alpha: 8-10 Hz in all the regions) were induced by MA task. Further evaluation of these changes showed a negative correlation between the change rates (with the control resting condition as the baseline) in fast alpha power and that in LF/HF. These results suggest that specific correlated relationships exist between the change in autonomic nervous activity and EEG power depending on the difference in mental task (i.e., meditation or MA).