Chemical reaction method using chemical reaction apparatus

A chemical reaction method comprising:

• preparing a chemical reaction apparatus including a horizontal flow reactor inside of which has been partitioned into multiple chambers continuously arranged in series by multiple partition plates, and a liquid content horizontally flows with an unfilled space being provided thereabove and continuous above the multiple partition plates in the reactor,
• 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,
• wherein one or more weirs that are flow paths of the content are formed in an upper portion of each of the partition plates;
• flowing the content over each of the multiple partition plates inside the reactor;
• irradiating the content flowing inside the reactor with microwaves;
• and
• changing an inclination angle of the reactor within a range in which, in each of the chambers, 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, when the content flows inside the reactor.