To provide a chemical reaction device capable of effectively irradiating the contents with microwaves even if the amount of the contents changes. A chemical reaction device provided with: a horizontal, flow-type reactor (13) which contains multiple chambers partitioned by multiple partition plates and in which the liquid contents flow in the horizontal direction in a state having an unfilled space above; a microwave generator (14) which generates microwaves; and one or more waveguide tubes (15) which transmit, in the unfilled space of the reactor (13), microwaves generated by the microwave generator (14). The reactor (13) has a shape such that, even when the height of the liquid surface changes depending on changes in the amount of the contents, the surface area of the liquid surface is constant.