A valve element mainly consists of silicon rubber and has a plate-like shape. The valve element is provided with an opening/closing section which opens and closes with insertion and removal of a long member. The opening/closing section has a first slit reaching only one surface of the valve element and a second slit reaching only the other surface of the valve element and intersecting within the valve element with the first slit. In the valve element, the inner surfaces of the first slit and the second slit are processed with plasma and reformed. When the valve element is in a natural state, the first slit and the second slit are open.
