Recent topics on eco-friendly metal nanoparticle preparation using plant extracts and sustainable metal nanoparticle preparation using waste materials was introduced. Furthermore, original experimental data on the preparation of antimicrobial silver nanoparticles using the waste leaves of Yamato peony (Paeonia lactiflora), a traditional medicinal plant cultivated in Nara, Japan, which is a World Heritage site, were presented. In the silver nanoparticle preparation using waste leaves of Yamato peony, after a 1-hour reaction at 90 °C of a mixture of silver nitrate solution with the extract obtained by hot water extraction of the leaves, UV-vis spectra of the solution showed a surface plasmon resonance peak at ca. 440 nm, indicating the formation of silver nanoparticles. The dynamic light scattering (DLS) measurement of the solution also showed peaks at around 10 and 100 nm, respectively. SEM observations and EDS analysis of the dried sample confirmed the presence of agglomerates of nanoparticles with a silver signal. The higher the reaction temperature, the narrower the particle size distribution was obtained. In addition, antimicrobial tests of the prepared silver nanoparticles against the model microorganisms Escherichia coli and Bacillus subtilis showed that the silver nanoparticles have antibacterial activity against both bacteria. In conclusion, antimicrobial silver nanoparticles can be prepared using the waste parts of crude drug plants for traditional medicinal use in Nara, Japan, without the emission of effluents containing organic solvents or strong chemical reducing agents.