A number of nanometer-sized holes, 20-80 nm wide and 0.6-0.7 nm deep, were produced at NH4F-etched, atomically nearly-flat n-Si(111) surfaces while platinum was electrochemically deposited at -0.40 V vs SCE in 5.0 x 10(-3) M H2PtCl6 + 0.5 M Na2SO4 of pH 2.9. The nano-holes were produced only during the Pt deposition at potentials more negative than -0.35 V. Experiments have shown that hole injection (or extraction of valence-band electrons) by H2PtCl6 plays no important role in the nano-hole formation. When the n-Si(111) surfaces were kept at -0.4 V in 0.18 M H2SO4 which has the same pH as the 5.0 x 10(-3) M H2PtCl6 + 0.5 M Na2SO4, much smaller nano-holes than the above case were produced, suggesting that electrochemical reductive dissolution of Si occurs in acidic solutions. It is proposed as a possible mechanism that deposited Pt atoms, while migrating along the n-Si surface, act as a catalyst for the reductive dissolution of Si, leading to acceleration of nano-hole formation.