Bioassay equipment which prevents a variation in concentration of a medium or deposition/bonding of a substance in the medium to be caused by drying of the medium being stored or held in a reaction region providing an inter-substance interaction field. The bioassay equipment (2) comprises at least a means for supplying a medium containing a substance pertaining to the interaction to the reaction region R providing the field of inter-substance interaction such as hybridization, and a means for supplying required water automatically. The bioassay equipment (2) may comprises a means for automatically detecting the volume of the medium held in the reaction region R.