A branching unit for boring without stopping passage of fluid, which is suitable for forming an opening at a pipe wall by boring a part of the pipe wall of an existing pipe using a drill having a hole saw to connect a branch pipe to the existing pipe, comprising: a sealing case including plural numbers of separate casings (21,22) separated in a circumferential direction of the existing pipe as well as surrounding a part of the existing pipe, and a branch pipe portion formed integrally with a first separate casing of the plural separate casings (21,22) as well as projecting and branching off in a radial direction of the existing pipe; a valve body having an arc shape or a circular shape in cross section which opens and closes a branch hole of the branch pipe portion by rotating along the circumferential direction of the existing pipe between an inner circumferential surface of the sealing case and an outer circumferential surface of the existing pipe; a valve seat formed at the inner circumferential surface in the first separate casing, the valve body touching the valve seat in a valve closing state; and a rotation mechanism for rotating the valve body, wherein the valve body includes a concave surface which is curved along the outer circumferential surface of the existing pipe and a convex surface facing the branch hole in the valve closing position, and the rotation mechanism includes an operation gear rotationally operated from an outside of the sealing case and a driven gear meshed with the operation gear, rotating in the circumference direction with the rotation of the operation gear to open and close the valve body, characterized in that the driven gear is formed on the convex surface of the valve body, and a contact surface touching the valve seat in the convex surface more protrudes than the surface of the driven gear in the radius direction of the existing pipe. a sealing case including plural numbers of separate casings (21,22) separated in a circumferential direction of the existing pipe as well as surrounding a part of the existing pipe, and a branch pipe portion formed integrally with a first separate casing of the plural separate casings (21,22) as well as projecting and branching off in a radial direction of the existing pipe; a valve body having an arc shape or a circular shape in cross section which opens and closes a branch hole of the branch pipe portion by rotating along the circumferential direction of the existing pipe between an inner circumferential surface of the sealing case and an outer circumferential surface of the existing pipe; a valve seat formed at the inner circumferential surface in the first separate casing, the valve body touching the valve seat in a valve closing state; and a rotation mechanism for rotating the valve body, wherein the valve body includes a concave surface which is curved along the outer circumferential surface of the existing pipe and a convex surface facing the branch hole in the valve closing position, and the rotation mechanism includes an operation gear rotationally operated from an outside of the sealing case and a driven gear meshed with the operation gear, rotating in the circumference direction with the rotation of the operation gear to open and close the valve body, characterized in that the driven gear is formed on the convex surface of the valve body, and a contact surface touching the valve seat in the convex surface more protrudes than the surface of the driven gear in the radius direction of the existing pipe.