A fuel injection device for diesel engines which dispenses with brackets for a common rail and for a fuel supply pump, enables shortening of a fuel pipe, is space-saving with fewer parts, is lightweight and has a good maintainability. A common rail is mounted to a cylinder body, and a fuel supply pump is mounted to overlap the common rail, so that a fuel pressure-fed by the fuel supply pump is accumulated by the common rail to be injected into a fuel chamber of a cylinder.