A holder configured to allow light transmission probes for emitting light and light reception probes for receiving light to be arranged alternately and be spaced apart from each other by a second set distance r2. The holder has a plurality of first through holes each provided at a position spaced apart by a first set distance r1 shorter than the second set distance r2 from a position of the retained light transmission probe or a position of the retained light reception probe. Each of the through holes of the plurality of first through holes allow reference probes for emitting or receiving light to be inserted thereto. One of the first through holes provided therein with none of the reference probes has a non-light transmissive attachment member in a detachable manner.
