A waveguide for tympanic thermometer formed by an elongate hollow member having substantially ellipsoidal shape truncated at both ends along its major axis direction. The waveguide is interposed between a tympanic membrane of which the temperature is to be measured and an infrared radiation sensor so as to guide the infrared radiation from the tympanic membrane to the sensor. Alternatively, the waveguide may also be a hollow member including an end portion having a gradually reduced inner diameter toward the side of sensor. The gradually reduced end portion is preferably of a truncated half ellipsoidal shape having a length between about 1/10 to about 1/3 of the distance from the sensor to the distal end of the waveguide.