A developing device of the present invention includes a developing roller provided with a plurality of magnetic poles. An electron emission unit is provided on a surface of the first substrate facing the second substrate, and a light emission unit is provided on a surface of the second substrate facing the first substrate. Each one of the pair of opposing sidewalls of the clip has a frontal portion and a rear portion. , to have zero output current at zero glucose concentration, even in the presence of other electroreactive species of serum or blood. Therefore, by virtue of the optical devices, an optical path can be determined by selecting a shift position of said optical device from nine shift positions.