A reaction method of performing an adsorption reaction in which a subject substance of analysis is specifically adsorbed in a first channel, the method includes: flowing a specimen liquid to a second channel connected to the first channel so that the specimen liquid is fed to the first channel, the specimen liquid containing the subject substance and a labeled substance that can be bonded to the subject substance; stopping feeding of the specimen liquid by detecting an event that a rear end of the specimen liquid flows into the first channel; joining a washing liquid to the rear end of the specimen liquid which stops in the first channel by flowing the washing liquid to a third channel that is converged to a connection portion of the second channel; and feeding the washing liquid to the first channel after the washing liquid is joined to the rear end.