A raised distal end portion of a strut portion is in a non-joined state to a back side of an upper duct wall portion and is made up of a spherical abutting and sliding portion which can be brought into abutment with the back side by virtue of point contact and which is allowed to slide relative to the back side. When a downward pressure is exerted on to the spherical abutting and sliding portion from the upper duct wall portion, the spherical abutting and sliding portion is caused to slide relative to the back side and a root portion of the strut portion is deformed at the same time, whereby the strut portion is allowed to incline and fall obliquely downwards.