A method, comprising the following (a) to (g): (a) providing a microchannel device comprising an upstream channel portion, a downstream channel portion, a liquid holding portion provided between a downstream end portion of the upstream channel portion and an upstream end portion of the downstream channel portion, and a gas bypass channel portion provided so as to bypass the liquid holding portion from the downstream end portion of the upstream channel portion to the upstream end portion of the downstream channel portion; (b) filling the gas bypass channel portion with a gas and holding a main liquid plug in the liquid holding portion; (c) causing a first upstream liquid plug and a second upstream liquid plug to sequentially flow through the upstream channel portion toward the liquid holding portion; (d) causing the first upstream liquid plug to flow into the liquid holding portion to merge the first upstream liquid plug with the main liquid plug, and pushing out a part of the main liquid plug after the merging to the downstream channel portion; (e) causing a gas following the first upstream liquid plug to flow into the gas bypass channel portion to push out a part of the gas in the gas bypass channel portion to the upstream end portion of the downstream channel portion, to thereby cut the part of the main liquid plug, which is pushed out in the item (d), to form a first downstream liquid plug; (f) causing the second upstream liquid plug to flow into the liquid holding portion to merge the second upstream liquid plug with the main liquid plug and pushing out a part of the main liquid plug after the merging to the downstream channel portion; and (g) causing a gas following the second upstream liquid plug to flow into the gas bypass channel portion to push out a part of the gas in the gas bypass channel portion to the upstream end portion of the downstream channel portion, to thereby cut the part of the main liquid plug, which is pushed out in the item (f), to form a second downstream liquid plug. (a) providing a microchannel device comprising an upstream channel portion, a downstream channel portion, a liquid holding portion provided between a downstream end portion of the upstream channel portion and an upstream end portion of the downstream channel portion, and a gas bypass channel portion provided so as to bypass the liquid holding portion from the downstream end portion of the upstream channel portion to the upstream end portion of the downstream channel portion; (b) filling the gas bypass channel portion with a gas and holding a main liquid plug in the liquid holding portion; (c) causing a first upstream liquid plug and a second upstream liquid plug to sequentially flow through the upstream channel portion toward the liquid holding portion; (d) causing the first upstream liquid plug to flow into the liquid holding portion to merge the first upstream liquid plug with the main liquid plug, and pushing out a part of the main liquid plug after the merging to the downstream channel portion; (e) causing a gas following the first upstream liquid plug to flow into the gas bypass channel portion to push out a part of the gas in the gas bypass channel portion to the upstream end portion of the downstream channel portion, to thereby cut the part of the main liquid plug, which is pushed out in the item (d), to form a first downstream liquid plug; (f) causing the second upstream liquid plug to flow into the liquid holding portion to merge the second upstream liquid plug with the main liquid plug and pushing out a part of the main liquid plug after the merging to the downstream channel portion; and (g) causing a gas following the second upstream liquid plug to flow into the gas bypass channel portion to push out a part of the gas in the gas bypass channel portion to the upstream end portion of the downstream channel portion, to thereby cut the part of the main liquid plug, which is pushed out in the item (f), to form a second downstream liquid plug.