We have developed a micro-chemical chip coupled with a post-wall waveguide for various chemical reactions monitored in situ during microwave irradiation at 24.125 GHz. This device has a structure in which a microchannel passes between metal post-walls. Fluorinated ethylene propylene (FEP) resin is used for the microchannel’s structure, and glass with an indium tin oxide (ITO) film is also used to prevent the leakage of microwaves from the top and bottom of the microchannel. FEP is a feasible material for use as a waveguide and a reactor structural material for chemical synthesis owing to its low dielectric loss, high heat resistance (up to 200 ℃), and chemical inertness. In addition, its excellent transparency in the visible region also allows the optical monitoring of various reactions during microwave heating. An ITO film was sputtered on the glass, and the resulting glass had a microwave transmittance of −27.5 dB. When water and ethylene glycol were heated with a microwave input power of 4.0 W, the temperature rose to 82.7 and 91.5 ℃, respectively. In addition, by observing the inside of the microchannel during microwave irradiation, it was possible to confirm the state of boiling induced in the vicinity of the microwave input section. This is consistent with the temperature distribution in the simulation results. This device enables the rapid heating of solvents by microwave irradiation and optical monitoring of the microwave-induced chemical reactions.