A biological signal measuring system includes: a light emitter emitting light beams having different N kinds of wavelengths, where N is an integer of four or more; a light receiver outputting N kinds of signals respectively in accordance with received light intensities of the N kinds of light beams that have been passed through or reflected from a living tissue; a first calculating section acquiring N kinds of light attenuations based on the N kinds of signals; a second calculating section acquiring (N−1) kinds of blood-derived light attenuations based on two light attenuations related to (N−1) kinds of combinations selected from the N kinds of light attenuations; a third calculating section identifying concentrations of (N−1) kinds of in-blood substances based on the (N−1) kinds of blood-derived light attenuations; and an outputting section outputting the identified concentrations.