A valve body formed from an elastic member in the form of an elliptical or circular film (disk shape) has upper and lower surfaces. A first concave portion is formed on the upper surface side of the valve body. The valve body further 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. The second concave portion is formed at a portion of the concave surface side of the valve body corresponding to an intersecting portion, and the third concave portions are formed at the opposite end portions of the first slit. The second concave portion extends, in a thickness wise direction of the valve body, to an intermediate portion of the first slit but not to the second slit.