In a method of manufacturing a glass interposer, first, stacked bodies formed on a front surface and a back surface of a glass substrate are processed along division lines (streets) to form first grooves having a first width and such a depth as not to reach the glass substrate, while leaving a residual resin portion at bottoms of the first grooves. Thereafter, the residual resin portion is subjected to ablation processing to expose the front surface and the back surface of the glass substrate, thereby forming second grooves having a second width narrower than the first width. A laser beam is applied along the division lines through the second grooves to form modified layers in the inside of the glass substrate, and an external force is exerted on the glass substrate to divide the glass substrate, with the modified layers as division starting points.