According to one embodiment, a nucleic acid reaction tool includes a support having a first surface, a covering body having a second surface, and a groove opened on the second surface, and a primer set. The covering body is in contact with the support to form a reaction space surrounded by the first surface and the groove. The groove includes, on an inner surface of the reaction space, rising surfaces opposed to each other, and a rear surface connecting one end of the side surfaces, and a primer fixing region to which the primer set is fixed, the primer fixing region being located at a corner where the one end of the side surfaces connected to the rear surface in the reaction space.