A three-way stopcock unit, a check valve, and a fluid substance transfer device, which are easily downsized, have a high yield, and are manufactured at low cost. The check valve in the three-way stopcock unit according to the present invention includes: a first housing having a valve seat in which an opening is provided a second housing inserted into the first housing and a valve element held between the valve seat of the first housing and an opening end portion of the second housing. The valve element has a substantially U-shaped cut line formed outside of an outer rim portion of the opening provided in the valve seat of the first housing, and inside of an inner rim portion of the second housing. A portion inside of the cut line is used as a switching valve, and the valve seat of the first housing and the opening end portion of the second housing are closed in a liquid-tight state at a portion outside of the cut line.
