In an apparatus for determining a concentration of a light absorbing substance in blood, a plurality of photo emitters are adapted to emit light beams having different wavelengths toward a living tissue including a blood vessel. A photo receiver is adapted to receive the light beams which have been transmitted through or reflected from the living tissue. A driver inputs driving currents for causing the respective photo emitters to emit the light beams. An optimizer obtains values of AC components of the light beams received by the photo receiver, and adjusts values of the driving currents such that the values of the AC components fall within a predetermined range.