A thermal cycler includes: a first mounting unit and a second mounting unit which are cylindrical; a temperature gradient forming unit which forms a temperature gradient along mounting directions of the first mounting unit and the second mounting unit; and a driving mechanism which rotates the first mounting unit, the second mounting unit, and the temperature gradient forming unit around a rotating shaft having a component perpendicular to a direction in which gravity is applied and a component intersecting the mounting directions of the first mounting unit and the second mounting unit, wherein the first mounting unit and the second mounting unit are disposed on opposite sides to each other with the rotating shaft interposed therebetween, the mounting direction of the first mounting unit and the mounting direction of the second mounting unit are in the same direction.