Abstract: A reflection density measuring system comprises an incubator in which a plurality of chemical assay slides can be accommodated and arranged in one plane, a light source disposed outside the incubator, and a measuring head connected to the light source by way of a fiber optic member. The measuring head is movable to be opposed to each of the chemical assay slides in the incubator under deformation of the fiber optic member, and the fiber optic member is formed of an optical fiber bundle at least at the portion at which the fiber optic member is deformed in response to movement of the measuring head.