A biological photometric device comprises a light irradiating unit for irradiating an object to be examined with a light having a predetermined wavelength and sympathizing with oxygenated hemoglobin and deoxygenated hemoglobin through irradiating optical fiber, a light detecting unit for detecting and amplifying the light passing through a detecting optical fiber and the object, a signal processing unit adapted for computing the hemoglobin time change information on the oxygenated hemoglobin, the deoxygenated hemoglobin and the total hemoglobin in the object from the signals detected by the light detecting unit, and having noise detecting means for arithmetically processing the detected signals and determining/detecting whether the time change information is the noise (low S/N noise) attributed to the presence of obstacles to passing of light between the object and the end face of the optical fiber or noise (mirror noise) attributed to the damage to the light irradiating unit, and a display unit for displaying the noise signals determined/detected by the signal processing unit to enable identification of the type of noise.