PROBLEM TO BE SOLVED: To lock an internal insertion portion in an insertion port into which the internal insertion portion of a probe or an endoscope is inserted from a distal end for the measurement of a reference sample and the like.SOLUTION: An internal structure is constructed to communicate with an insertion port 20 and form a hole 22 having a marginal portion in which an distal end of an internal insertion portion 3 makes contact with and stops in a deep end portion of the insertion port, and a reference sample 4 is placed on a tip of the hole. A locking structure is provided with locking members (1A-1G) which are composed to fixed to or detachable from the insertion port, wherein the locking member fixed to the insertion port is elastically modified to receive the internal insertion portion to be inserted in the insertion port, and the locking member makes pressure contact with an outer peripheral surface of the internal insertion portion inserted in the insertion port by the elastic force of the locking member to lock the internal insertion portion. For example, the locking member is composed of an elastomer material and is arranged so as to cover the insertion port, wherein the locking member forms a wall portion, in which a slit is formed, and constructs a hole portion with an inner diameter smaller than the outer diameter of the internal insertion portion. There is an example that applies an O-ring as the locking member, too.
