A chemical reaction method having steps of preparing a chemical reaction apparatus by partitioning an inside of a horizontal flow reactor into multiple chambers by multiple partition plates, and flowing a liquid horizontally with an unfilled space being provided thereabove, generating microwaves with a microwave generator, and transmiting the microwaves, with at least one waveguide, to the unfilled space in the reactor. Also forming a top portion of the partition plates act as a weir, inclining the reactor such that, in each of the chambers, a weir height on the inlet side is higher than a weir height on the outlet side by at least an overflow depth at the partition plate on the outlet side, flowing content over each of the partition plates inside the reactor, and configuring the weir heights of the partition plates in the reactor are the same in a state where the reactor is not inclined.

A chemical reaction apparatus includes: a horizontal flow-type reactor inside of which has been partitioned into multiple chambers by multiple partition plates, and a liquid content horizontally flows with an unfilled space being provided thereabove; a microwave generator that generates microwaves; and at least one waveguide that transmits the microwaves generated by the microwave generator to the unfilled space in the reactor. The content flows over each of the partition plates, and, in each chamber, a weir height on an inlet side is higher than a weir height on an outlet side by at least an overflow depth at the partition plate on the outlet side.