A thermal cycler includes a holder receiving a reaction chamber including a channel filled with reaction mixture and an immiscible liquid having different specific gravities and allowing the reaction mixture to move along an opposed inner wall, a temperature gradient forming unit forming a temperature gradient in the movement direction of the reaction mixture relative to the channel during loading into the holder, and a driving unit rotating the holder and temperature gradient forming unit about a rotation axis having a component perpendicular to the gravitational direction and a component perpendicular to the movement direction of the reaction mixture, and a maximum distance from the rotation axis to a point in the channel is smaller than a maximum distance connecting two points in the channel when projected on a plane perpendicular to the rotation axis.