In this study, we have developed a basic system of an ingestible core body thermometer powered by a gastric acid battery. A custom integrated circuit (IC) with voltage boosting, coding, modulating and transmitting functions was prototyped. A small gastric acid battery based on a pair of Mg and Pt thin-film electrodes was also fabricated by microfabrication technology. Charge storage in a storage capacitor was succeeded with the battery and IC at the boosted voltage of about 3 V. Then, an evaluation board of this system was prepared. The footprint for mounting the essential electrical components was reasonably small, which suggested the possibility to miniaturize this system to a tablet size. Magnetic-field coupling telecommunication of measured temperature data between a small transmission coil and a loop antenna as a receiver was successfully demonstrated. The power consumption for one time temperature measurement and telecommunication measured 10∼30 pJ. These results proved the feasibility of the ingestible thermometer system.