Yellow Cameleons are genetically encoded Ca2+ indicators in which cyan and yellow fluorescent proteins and calmodulin work together as a fluorescence (Förster) resonance energy transfer Ca2+-sensor probe. To achieve ultrasensitive Ca2+ imaging for low resting Ca2+ or small Ca2+ transients in various organs, we generated a transgenic mouse line expressing the highest-sensitive genetically encoded Ca2+ indicator (Yellow Cameleon-Nano 15) in the whole body. We then focused on the mechanism of exocytotic events mediated by intracellular Ca2+ signaling in acinar cells of the mice with an agonist and observed them by two-photon excitation microscopy. In the results, two-photon excitation imaging of Yellow Cameleon-Nano 15 successfully visualized intracellular Ca2+ concentration under stimulation with the agonist at nanomolar levels. This is the first demonstration for application of genetically encoded Ca2+ indicators to pancreatic acinar cells. We also simultaneously observed exocytotic events and an intracellular Ca2+ concentration under in vivo condition. Yellow Cameleon-Nano 15 mice are healthy and no significant deteriorative effect was observed on physiological response regarding the pancreatic acinar cells. The dynamic range of 165% was calculated from Rmax and Rmin values under in vivo condition. The mice will be useful for ultrasensitive Ca2+ imaging in vivo.