Three-way stopcock, and liquid transfusion circuit or blood transfusion circuit using the three-way stopcock

One kind of the three-way stopcock which can be opened to the conventional method of operating the same manner as on / off, and can reduce stagnant fluid portion of the fluid flow channel. The opening in the branch, the first branch and a second branch aperture openings are arranged on a straight line, the third branch opening is arranged in a direction perpendicular to the straight line connecting the first and second branch opening. The flow channel is provided with a conversion portion disposed on a straight line in a first flow passage opening and a second flow path opening, and a well disposed in a direction perpendicular to the straight line connecting the first and second branches of the third opening opening the flow path.