A radiation thermometer measures temperature of the eardrum. The thermometer comprises a signal processor for calculating a temperature from an output of a light receptor, which receives only the infrared rays radiated directly from the eardrum and/or vicinity of it, and a notification system for notifying the temperature resulted from the calculation. Since the structure does not receive an influence of any radiant heats from other than those of the eardrum and/or vicinity of it, temperature change of a probe does not become a factor of measuring errors, so as to offer an accurate measurement of temperatures. An infrared-receiving element is positioned within a triangle configured by an intersection between a light path and an optical axis, and two image points of hypothetical end points formed by an optical condenser, when viewed in a cross sectional plane including the optical axis of the optical condenser, where (a) the light path is a path that extends from the hypothetical end point to the image point of the hypothetical end point formed by the optical condenser by passing through a rim of the optical condenser on the same side as the hypothetical end point with respect to the optical axis, and (b) the hypothetical end point is a point at which a straight line drawn from the rim of the optical condenser toward the probe in a manner to be tangent to an inner surface of the probe on the same side as the rim of the optical condenser with respect to the optical axis crosses a plane at a tip of the probe.