A valve body for a medical device includes a base including a first surface and a second surface and slits provided in the base. The slits include a first slit and a second slit. The first slit extends from the first surface toward the second surface by a first length. The second slit extends from the second surface toward the first surface by a second length. The at least a part of each of the first slit and the second slit are connected at a connecting section. When the introduction member is introduced into a living organism, the introduction member passes through the connecting section and moves from the first surface side toward the second surface side. In a single state in which the valve body is not assembled with the medical device, a gap of the second slit is smaller than a gap of the first slit.
