A vent duct having an inner fluid flow passage includes a bent portion formed to a duct body portion at which the fluid flow passage is bent, and a protruded portion formed on a downstream side of the bent portion so as to protrude from an inner peripheral surface portion toward an axial center of the fluid flow passage, wherein the protruded portion includes an upstream side end portion formed to be gently continuous to the inner peripheral surface portion of a downstream side portion of the bent portion, a body portion extending from the upstream side end portion toward the downstream side portion along the fluid flow passage, and a downstream side end portion along the fluid flow passage to be continuous to the body, the downstream side end portion of the protruded portion being formed to be perpendicular to the inner peripheral surface portion.