PURPOSE: To capture such as iron scales to prevent erosion by providing a lead-in part where chamfering with radius R is applied to the inner face of a part of the mother pipe where steam flow in the mother pipe branches to a safety valve side via a nozzle neck, and providing an inner diameter contraction part in the hole of the nozzle neck fro introducing the branched steam.

CONSTITUTION: Chamfering with radius R for a lead-in part 7 is applied in an angular range from parallel direction to advancing direction of steam flow in the mother pipe 3 to less than nearly 45° in relation to the steam flow. As the steam flow passing through the mother pipe 3 produces lifting vortex stream in a nozzle neck 2 by means of the lead-in part 7 where the inner face of the mother pipe 3 at a branching part from the mother pipe 3 toward a safety valve 1 via the nozzle neck 2 is subjected to chamfering with radius R iron scales 12 are forcibly turned, and lifted along the inner wall of the nozzle neck 2 by centrifugal force. Steam leakage and/or erosion can be prevented by catching these iron scale is 12 into an annular void 8 between the outer pipe 9 and the inner pipe 10 of the nozzle neck 2 before they reach the inside of the safety valve 1.