To improve an internal pressure fatigue strength by a method wherein a branch hole on the common rail side forms an oval hole having a large diameter in the direction of the diameter of a flow passage and the branch hole is caused to coincide with the center of the flow passage of a common rail.

The interior of the axis of a main pipe rail 1 being a common rail forms a flow passage 1-1 in a circular shape in cross section and a gap is held at a peripheral wall part in an axial direction. Further, a branch hole 1-2 consisting of an oval hole communicated with a flow passage 1-1 and having a peripheral surface opened externally and forming a pressure receiving seat surface 1-3 is formed on the central line of the flow passage 1-1. A branch hole 1-2 consists of an oval hole having a long diameter in the direction of the diameter of the flow passage 1-1, in a circular shape in cross section, of a main pipe rail 1. The pressure receiving seat surface 1-3 forms an approximately conical surface in relation of connection of the press seat surface having an approximately conical surface. This constitution relieves a stress generated at the opening end part of the common rail of a branch hole, whereby the internal pressure fatigue strength of the branch hole is increased.