A microfluidic device includes a flow path through which a reaction solution flows, and an introducing portion for introducing the reaction solution into the flow path, wherein the flow path passes alternately and repeatedly several times through a first temperature region and a second temperature region having different predetermined temperatures, and the flow path includes the repeating unit portions passing through the first temperature region and the second temperature region, the repeating unit portions having decreasing lengths as the repeating unit portions are located farther away from the introducing portion.