Glass layers 3 are molten by irradiation with laser light L, so as to generate a temperature difference in a part along regions to be fused R in each of glass substrates 40, 50 so that main faces 40a, 50a on the glass layer 3 side have a temperature higher than that on main faces 40b, 50b on the opposite side of the glass layers 3. Thereafter, the molten glass layers 3 solidify, and cooling generates a stress in the glass substrates 40, 50. Here, since initial fractures 8 are formed in the glass substrate 50 so as to overlap the glass layers 3, fractures grow in the thickness direction of the glass substrates 40, 50 through the glass layers 3 from the initial fractures 8 acting as a start point.