A through-hole forming method includes steps of forming a first impurity region (102a) around a region where a through-hole is to be formed in the first surface of a silicon substrate (101), the first impurity region (102) being higher in impurity concentration than the silicon substrate (101), forming a second impurity region (102b) at a position adjacent to the first impurity region (102a) in the depth direction of the silicon substrate (101), the second impurity region (102b) being higher in impurity concentration than the first impurity region (102a), forming an etch stop layer (103) on the first surface, forming an etch mask layer (104) having an opening on the second surface of the silicon substrate (101) opposite to the first surface, and etching the silicon substrate (101) until at least the etch stop layer (103) is exposed via the opening.