A conventional living body information acquiring apparatus for measuring a skinfold thickness of a living body could not fix a relative position of skinfold thickness measuring means to the living body. Thus, the present invention comprises skinfold thickness measuring means of measuring a skinfold thickness of a living body 9 composed of a light source 1 and a photodetector 2, a protruding part 3 as relative position fixing means of fixing a relative position of living body information acquiring means to the living body 9 substantially constant, and holding means 4 of holding the light source 1, the photodetector 2, and the protruding part 3.