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. A first concave portion 64 having a bottom face formed as a concave face 641 is formed on the upper face 601 side of the valve body 6. The valve body 6 further 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. The second concave portion 65 is formed at a portion of the concave face 641 side of the valve body 6 corresponding to an intersecting portion 63, and the third concave portions 66 are formed at the opposite end portions of first slit 61. The second concave portion 65 extends, in a thicknesswise direction of the valve body 6, to an intermediate portion of the first slit 61 but not to the second slit 62.
