A valve body is formed from an elastic member in the form of an elliptic or circular film (disk shape) having an upper face and a lower face. The valve body has an opening and closing section which is opened and closed in response to insertion and pulling out of a dilator and is formed from a first slit, a second slit, a second concave portion and two third concave portions. A pair of ribs and six projections are formed on the lower face side of the valve body. The ribs are disposed in opposing relationship to each other with the second slit interposed therebetween and extend along the lower face. Further, the projections are disposed such that they are positioned on the inner side of the ribs and are opposed to each other with the second slit interposed therebetween.