A bioassay substrate (1) has a circular, planar major surface similar to that of an optical disc, e.g. a CD. The substrate (1) is rotary-driven about a central hole (2). The substrate (1) is provided, on the surface (1a) thereof, with a plurality of wells (8) where hybridization reaction takes place between a probe DNA (nucleotide chain for detection) and a sample DNA (target nucleotide chain). A transparent electrode layer (4) is formed beneath the well (8) in the substrate (1). At the time of hybridization, an external electrode (18) is brought closer from the upper surface (1a) side of the substrate (1) and an AC power is fed between the transparent electrode layer (4) and the external electrode (18), thus applying an AC field to the substrate (1) in the vertical direction.