The present invention can realize an electronic clinical thermometer of longlife, low cost and high accuracy. The device is formed of a thermistor, a reference resistor element, a voltage changeover switch for changing over applied voltage to charge a capacitor via the thermistor or the reference resistor element, an A/D converter to output an ON signal during the time interval when the condenser is keeping a voltage higher than the prescribed value by detecting the variation of voltage due to discharging the electricity stored in the capacitor via the thermistor or the reference resistor element, a timer for measuring the discharge duration by measuring the time the ON signal lasts, and a calculating section for calculating the ambient temperature of the thermistor using the average value of the discharge duration of the electricity stored in the capacitor via the thermistor, and each discharge duration of discharging stored electricity in the capacitor via the reference resistor element at the moments immediately before and after the discharge.依照本發明可實現壽命長、廉價而具有高計測精度的電子體溫計。本發明之電子體溫計包含有熱敏電阻，基準電阻體，透過該熱敏電阻或該基準電阻體而積蓄電荷於供電容器使用可切換施加電壓的電壓切換部，檢出透過該熱敏電阻或該基準電阻體積蓄於該電容器之電荷放電時隨著放電之電壓變化，可在該電容器具有所定電壓以上之電壓期間，輸出ON信號的A/D變換部，藉計測ON信號之時間而計測放電時間的計時器，及利用放出透過該熱敏電阻積蓄於該電容器的電荷時的放電時間，與在該放電之前及之後，放出透過該基準電阻體積蓄於該電容器之電荷之各自放電時間的平均值，以算出該熱敏電阻周圍溫度的演算處理部。
