In this study, we have successfully demonstrated an in vivo telemetry of a tablet-shaped ingestible core-body thermometer with a gastric acid battery. A pair of Mg and Pt plates was chosen as the negative and positive electrodes of the gastric acid battery, respectively. These plates and electrical components were mounted on two print circuit boards (PCB)s connected with a flexible substrate, and compactly packaged with a resin. Consequently, a tablet-shaped device of diameter 9 mm and height 7 mm was prototyped. The device successfully operated in the stomach of a dog due to a gastric acid power generation, measured the temperature, and transferred the data to a loop antenna as a receiver outside the body via magnetic-field coupling telecommunication. This achievement paves the way to realizing a safe and ingestible core-body thermometer for daily use.