A valve body 6 is formed from an elastic member in the form of an elliptic or circular film (disk shape) having an upper face 601 and a lower face 602. The valve body 6 has an opening and closing section 60 which is opened and closed in response to insertion and pulling off of a dilator 10 and is formed from a first slit 61, a second slit 62, a second concave portion 65 and two third concave portions 66. A pair of ribs 68 and six projections 69 are formed on the lower face 602 side of the valve body 6. The ribs 68 are disposed in an opposing relationship to each other with the second slit 62 interposed therebetween and extend along the lower face 602. Further, the projections 69 are disposed such that they are positioned on the inner side of the ribs 68 and are opposed to each other with the second slit 62 interposed therebetween.
