A silicon direct bonding (SDB) method by which void formation caused by gases is suppressed. The SDB method includes: preparing two silicon substrates having corresponding bonding surfaces; forming trenches having a predetermined depth in at least one bonding surface of the two silicon substrates; forming gas discharge outlets connected to the trenches on at least one of the two silicon substrates to vertically penetrate the bonding surface; cleaning the two silicon substrates; closely contacting the two silicon substrates to each other; and thermally treating the two substrates to bond them to each other. The trenches are formed along at least a part of a plurality of dicing lines, and both ends of the trenches are clogged. Gases generated during a thermal treatment process can be smoothly and easily discharged through the trenches and the gas discharge outlet such that a void is prevented from being formed in the junctions of the two silicon substrates due to the gases.