A biological signal measuring system includes: a light emitter emitting a first light beam and a second light beam； a light receiver outputting first and second signals in accordance with light intensities of the first and second light beams that have been passed through or reflected from a living tissue of a subject； a first calculating section acquiring a light attenuation of the first and second light beams based on the first and second signals； a second calculating section acquiring a blood-derived light attenuation based on the light attenuation of the first and second light beams； a third calculating section identifying a blood fill time for filling the living tissue, based on a temporal change of the blood-derived light attenuation associated with pressurization of the living tissue； and an outputting section outputting the identified blood fill time.
