A fiber unit (310) has an illumination fiber (311) and a plurality of light receiving fibers and is provided in a measurement probe (3) that performs optical measurement by inputting and outputting light from a distal end thereof. The illumination fiber (311) and the plurality of detection fibers include a soft portion (331) that is bendable, a distal end hard portion (332) having a greater hardness than the soft portion (331), and a pitch conversion portion (333) that connects the soft portion (331) and the distal end hard portion (332) and changes a distance between adjacent fibers at the distal end hard portion (332) with respect to a distance between the adjacent fibers at the soft portion (331), by bending and extending. When a most inclined angle of angles each formed by a central axis of the distal end hard portion (332) and a line segment connecting an end portion of the pitch conversion portion (333) closer to the soft portion (331) and an end portion of the pitch conversion portion (333) closer the distal end hard portion (332) is denoted by Deg_max, the following is satisfied: 1.0 < Deg_max < 6.0.
