A microfluid device (1) comprises a flow channel (100) in which a reaction solution (200) flows, and an introduction part (120) for introducing the reaction solution (200) into the flow channel (100). The flow channel (100) is configured so as to pass alternately through a first temperature region and a second temperature region, which are set to different prescribed temperatures, multiple times repeatedly. The flow channel (100) is configured so that the lengths of the first temperature region and second temperature region repeating unit portions shorten with increased distance from the introduction part (120).
