A bioassay substrate (1) takes a flat-plate shape in which the principal surface similar to that of optical disc such as CD, etc. is circular. At the center of the substrate (1), there is formed a center hole (2) into which a chucking mechanism for rotation and holding is inserted. The substrate (1) is rotationally driven with the center hole (2) being as center. On the substrate (1), there are formed two regions of a recording region (3) and a reaction region (4) which are formed in concentrical form in a radial direction. The recording region (3) is a region where, similarly to the optical disk information recording medium, laser beams are irradiated so that recording/reproduction of information is optically performed. The reaction region (4) is a region serving as the filed of mutual reaction between probe DNA (nucleotide chain for detection) and sample DNA (marked or labeled nucleotide chain), in concrete terms, the field of hybridization reaction.