A substance concentration monitoring method includes inserting a temperature probe noninvasively into a volume of a body in which a concentration of a substance is to be measured. An internal temperature of the volume is measured and an internal temperature signal is produced. An incident first near infrared beam in a first wavelength band is directed from a light source into a portion of the volume, the first wavelength band including a wavelength at which an absorption peak exists in a NIR absorption spectrum of the substance. An incident second NIR beam in a second wavelength band is directed from the light source into the portion of the volume, the substance exhibiting no absorption or negligible absorption in the second wavelength band. Substance concentration is calculated based on values corresponding to the internal temperature signal, a first and second initial power signal, and a first and second material absorption signal.
