Abstract: There are provided a microwave irradiation apparatus and a production method of metal nanoparticles for producing the metal nanoparticles that allow efficiently preparing the metal nanoparticles having small and uniform particle sizes. The present disclosure relates to the microwave irradiation apparatus that includes two microwave irradiation sources, two waveguides, and one reaction portion. The two microwave irradiation sources are disposed such that respective microwave emission units thereof face one another. The two microwave irradiation sources, the two waveguides, and the one reaction portion are disposed such that respective microwaves emitted from the two microwave irradiation sources pass through the two waveguides and contact an entire surface of the one reaction portion. Furthermore, the reaction portion is adjusted to have a specific size.