A rod-shaped capsule (diameter 21mm, length 7mm) containing quartz crystal resonator was developed for a telemetrical temperature sensor for the humer organs. An ultrasound probe emits 40 kHz ultrasound for 0.4 second and receives the reflected weve from the crystal for 0.6 second repetitively. The temperature is detected by the chance of the resonant frequency.The absolute accuracy of the measured temperature is 0.1℃ and the temperature resolution is 0.01℃.The sensor was swallowed in the stomach of a volunteer. The stomach temperature was obtained three times separately in different conditions with the ultrasonic probe placed on the abdominal wall above the stomach. The intragastric temperature changes following alternative cold and hot water intakes could be detected.The organ temperature can be measured without electro-magnetic interference and safely monitored by this technique without giving any discomfort to the subject after the sensor is placed. This capsule can be introduced into the body via digestive or urinary tract using a tubing, via blood vessel using a catheter without surgical meneuver, or can be placed surgically.