A simple always-portable stethoscope for enabling accurate diagnosis. A radiation/light-receiving fiber (11) serving as a probe part for noninvasively irradiating a diseased part with near-infrared light is applied to the diseased part so as to measure, e.g., a change of the cerebral circulation blood flow. The change is hard as sound pulse modulation to examine the change of the cerebral function. For example, a light beam of three wavelengths =760, 800, 830 nm from a semiconductor laser light source (22) is applied to the diseased part, the reflection data from the diseased part is processed by a control device (21), and the doctor can make a diagnosis with the doctor's ears by hearing with a receiver (32) the change as the change of the frequency of the sound the pitch and volume of which are constant.