PURPOSE: To process a non-porous Si substrate and to bore holes to penetrate the substrate from the substrate surface to the substrate rear in an arbitrary form and in an arbitrary size.

CONSTITUTION: A method of processing an Si substrate has a process for bringing partially a non-porous Si substrate 51 into a porous state from the substrate surface to the substrate rear at one place or more and a process for etching selectively and removing porous Si regions 53 only by wetting the substrate, in which non-porous Si regions 51 and the porous Si regions 53 are mixed, in a buffered hydrofluoric acid or a mixed solution obtainable by adding at least one kind of an alcohol and hydrogen peroxide water to the buffered hydrofluoric acid.