By using gold (Au) nanoparticles (NPs) as an optical near-field source under far-field illumination in combination with a silver (Ag) ion solution containing a photoinitiator, we coated Ag on Au NPs using a near-field (NF)-assisted process. We evaluated the change in the size of the NPs using transmission electron microscopy. Evaluation of the synthesized Ag volume over illumination power confirmed the squared power dependence of the NP volume with illumination using 808 nm light, i.e., a wavelength longer than the absorption edge wavelength of the photoinitiator molecules. The rate of volume increase was much lower than the plasmonic field enhancement effect. Therefore, the squared power dependency of the volume increase using a wavelength longer than the absorption edge wavelength originated from NF-assisted second-harmonic generation and the resulting excitation.