A biological optical measuring instrument comprising a measuring probe (101) for collecting light from a plurality of portions of a subject (214) transmitted through the subject (214) by means of an optical fiber (108) by guiding light emitted from a light source (102) by an optical fiber (107), and irradiating the light to the subject (214) so as to create a living body transmitted light intensity image of the subject (214) from the transmitted and collected light. The measuring probe (101) further comprises optical fiber fixing members (201, 210, 211) for fixing the optical fibers (107, 108) at a predetermined interval and support members (202, 204, 205) for rockably supporting the optical fiber fixing members. Thus, it is possible to provide a technique of performing living body optical measurement while the living body lies in lateral decubitus.