ATP is indispensable for all living organisms. Despite its importance, however, distribution and dynamics of ATP level inside living cells still remained unclear. We have developed FRET-based ATP biosensors, “ATeam” and “GO-ATeam”, which enabled us to visualize intracellular ATP level. Using these biosensors, we have successfully evaluated the effect of IF1 protein and Ca²⁺ on regulation of ATP level. Also, we found that replication of hepatitis C virus genome resulted in the declination of cytosolic ATP level of host cells and the formation of putative virus replication sites with high ATP levels.