PURPOSE: To improve measurement sensitivity or reliability in a method for measuring DNA or RNA fragment by the fluorescent polarization method.

CONSTITUTION: The DNA or RNA inside a specimen and an immobilization reagent where the DNA or RNA with a similar base arrangement as the DNA or RNA inside the specimen are competitively reacted with a DNA or RNA probe which has a base arrangement being complimentary to the DNA or RNA inside the specimen and is fluorescence-labeled, thus forming two chain DNA or RNA. The change between the degree of fluorescent polarization before forming two chains and that after forming two chains is measured, thus measuring the base arrangement corresponding to the DNA or RNA probe which exists in the DNA or RNA inside the specimen. Since the effective molecular weight change due to the complimentary connection reaction between the DNA or RNA which is fluorescence-labeled containing the base arrangement being complimentary to a measurement target and the immobilization DNA or RNA is large, the change in the degree of fluorescent polarization increases, thus improving the measurement sensitivity and reliability.