Measuring core body temperature is important in the study of human body temperature regulation in daily life. We measured core body temperature continuously using an ingestible capsule sensor that has excellent ambulatory utility in daily life. Daily temperature changes, including temperature increase during and just after bathing and temperature decrease during sleep, were observed in all subjects. Temperature readings and communication quality were found to be negligibly affected by the intracorporeal position of the capsule as determined by radiography, with no significant temperature difference among positions in the stomach, the small intestine and the large intestine. However, intake of hot or cold beverages during measurement should be avoided for accurate assessment. Loss of data from inside to outside the body was 3.7± 2.5% (1.4 ± 3.8% excluding sleeping hours). The increase in data loss during sleep was due to the change in position of the receiver. A loss of 0.66 ± 0.1% was obtained by placing the receiver less than 50 cm from the navel including during sleep, except during the first ten minutes after swallow. The path loss from inside to outside the body was estimated to be less than that of the capsule endoscope.