Au-Ag bimetallic catalysts on SiO2 were prepared using a simultaneous solid grinding (simul-SG) method, in which multiple kinds of organometallic complexes are ground with a support powder. After optimizing the Ag/Au ratio, the obtained catalysts exhibited remarkably high activity for low-temperature CO oxidation compared with that of Au/SiO2 or Ag/SiO2 catalysts. A detailed structural analysis using electron microscopy revealed that a higher ratio of Ag prevents growth of the particles via sintering and induces high densities of active Au-Ag contacts. Our simul-SG method is a promising tool for the easy preparation of multicomponent catalysts on various supports.