A microreactor, comprising in a plate like chip: a reagent storage section; the reagent storage section including: a storage chamber for storing a liquid reagent (57, 57a, 57b) therein; an inlet through which a driving solution is injected into the storage chamber; and an outlet through which the reagent (57, 57a, 57b) is pushed out of the storage chamber by the driving solution having been injected, a reagent feed-out flow path (43, 43a, 43b) which communicates with the reagent storage section and through which the reagent (57, 57a, 57b) is fed out of the reagent storage section; an opening (32, 32a to 32k) which communicates with the inlet of the reagent storage section through a flow path (45, 53a, 53b) and is configured to communicate with an external pump for feeding the driving solution when the external pump is connected to the chip; a branch flow path which is branched off, at a branch section at a position of the inlet, from the flow path (45, 53a, 53b) connecting the opening (32, 32a to 32k) to the inlet; an air evacuation flow path (46, 46a, 46b) which is provided in the branch flow path, and whose distal end is open to an outside, a water-repellent valve (51, 51a, 51b) which is provided at the outlet of the reagent storage section; and a water-repellent valve (51, 51a, 51b) which is provided at the inlet of the reagent storage section; wherein a flow path resistance of the air evacuation flow path (46, 46a, 46b) when a liquid flows therethrough is greater than a flow path resistance of a flow channel, which starts from the branch section as a starting point and includes from the branch section to the reagent feed-out flow path (43, 43a, 3b), when the liquid flows through the flow channel, so that after the fed driving solution has removed air existing between the reagent (57, 57a, 57b) and the driving solution through the air evacuation flow path (46, 46a, 46b) and the branch flow path has been filled with the driving solution, the driving solution pushes out the reagent (57, 57a, 57b) filled in the reagent storage section therefrom into the reagent feed-out flow path (43, 43a, 43b) while allowing no air to be entrapped between the driving solution and the reagent (57, 57a, 57b). a reagent storage section; the reagent storage section including: a storage chamber for storing a liquid reagent (57, 57a, 57b) therein; an inlet through which a driving solution is injected into the storage chamber; and an outlet through which the reagent (57, 57a, 57b) is pushed out of the storage chamber by the driving solution having been injected, a storage chamber for storing a liquid reagent (57, 57a, 57b) therein; an inlet through which a driving solution is injected into the storage chamber; and an outlet through which the reagent (57, 57a, 57b) is pushed out of the storage chamber by the driving solution having been injected, a reagent feed-out flow path (43, 43a, 43b) which communicates with the reagent storage section and through which the reagent (57, 57a, 57b) is fed out of the reagent storage section; an opening (32, 32a to 32k) which communicates with the inlet of the reagent storage section through a flow path (45, 53a, 53b) and is configured to communicate with an external pump for feeding the driving solution when the external pump is connected to the chip; a branch flow path which is branched off, at a branch section at a position of the inlet, from the flow path (45, 53a, 53b) connecting the opening (32, 32a to 32k) to the inlet; an air evacuation flow path (46, 46a, 46b) which is provided in the branch flow path, and whose distal end is open to an outside, a water-repellent valve (51, 51a, 51b) which is provided at the outlet of the reagent storage section; and a water-repellent valve (51, 51a, 51b) which is provided at the inlet of the reagent storage section; wherein a flow path resistance of the air evacuation flow path (46, 46a, 46b) when a liquid flows therethrough is greater than a flow path resistance of a flow channel, which starts from the branch section as a starting point and includes from the branch section to the reagent feed-out flow path (43, 43a, 3b), when the liquid flows through the flow channel, so that after the fed driving solution has removed air existing between the reagent (57, 57a, 57b) and the driving solution through the air evacuation flow path (46, 46a, 46b) and the branch flow path has been filled with the driving solution, the driving solution pushes out the reagent (57, 57a, 57b) filled in the reagent storage section therefrom into the reagent feed-out flow path (43, 43a, 43b) while allowing no air to be entrapped between the driving solution and the reagent (57, 57a, 57b).