A biomedical optical measurement apparatus comprising a light source unit for generating an inspection light containing multiple lights modulated at different frequencies, a light-receiving unit for receiving the light generated at said light source unit and passing through an object to be examined and for outputting the electric signals with the intensity corresponding to the received inspection light, and a detection means for detecting a signal with the same frequency of the reference signal in the output from said light-receiving unit. The detection means comprises an analog-digital conversion means for outputting digitized data by converting an input signal to a digital signal, a storage means for storing digitized data of multiple reference signals, a digital multiplication means for multiplying digitized data of input signals outputted from the analog-digital converting means by the digitized data of the reference signals read out from the storage means and for outputting the product of multiplication, and a digital band-limitation means for taking out DC data from the output from the digital multiplication means. Reference signal generating circuits of a number equal to that of frequencies of detected signals, which has been necessary for the conventional instrument, can be replaced by a single memory means and the configuration of the instrument can be simplified. Changes in frequency can be easily coped with by only re-writing the data of the storing means.