The invention relates to a method and apparatus for measuring the optical state on the inside of a reaction vessel. The apparatus includes: linking portions that optically connect with the interior of the reaction vessel that is linkable with reaction vessels, a connecting end arranging body having an arranging surface that arranges and supports along a predetermined path, connecting ends to which is provided back ends of the light guide portions, the front ends thereof being provided to the linking portions, the connecting ends being provided to the linking portions; a measurement device having measuring ends that are successively optically connectable with the connecting ends, and in which light based on the optical state is receivable by optical connections between the connecting ends and the measuring ends; and a light guide switching mechanism that successively optically connects the connecting ends and the measuring ends.