We have investigated the method of fabricating microstructures on a Si surface that is covered with a patterned gold (Au) mask, by electrochemical etching (anodization) in dilute hydrofluoric acid (HF) solution. We found that at electrode potentials below approximately 0.5 V, the Si surface is preferentially etched on the fringe of the Au mask, where a number of pores are formed. At higher electrode potentials, Au microwires with about 1 µm in width form along the fringe of the Au mask overlayer. We suggest that electromigration of Au towards the fringe of the Au mask induces self-assembling of Au atoms to form microwires. The observed self-formation of metal microwires would be beneficial to the fabrication of metal micro- or nano-structures on Si.