A new model explaining the energy supply mechanism to self-induced sloshing in a tank with circulating flow is proposed. The oscillation energy is considered to be supplied by pressure fluctuations caused by the interaction between the circulating flow and the sloshing motion. When the direction of the under-water-surface stream of sloshing motion coincides with the steady circulation, the dynamic pressure increases and the ensuing static pressure reduction causes an imbalance between the fluid and the atmosphere. If the static pressure reduces more in the region where the free surface is descending, the work done by the imbalance supplies energy to the sloshing oscillation.