To provide a three-way cock, where a patient himself can switch flowing channels easily and accurately and an insoluble substance is made hard to clog, in the three-way stopcock used in a peritoneal dialysis circuit.

The three-way cock 1 comprises a main body 2 having three branching tubes 4 on the outer periphery of a cylindrical part 5, and a cock 3 which is rotatably fit to the inside of the main body 2 and is provided with flow paths 10 corresponding to the three branching tubes 4 and having a barrel part 6, and a lever 7 is provided in the cock 3. The barrel part 6 is provided with a protrusion 9 on the outer periphery thereof, and the cylindrical part 5 is provided with a recess 8 which can receive the protrusion 9. The protrusion 9 has a saw blade shape which reduces in the rotational direction and can rotate only in one direction. The barrel part 6 is provided with three flow paths 10 corresponding to three branching tubes 4, and the flow paths 10 join together within the barrel part 6. The joined flow path 11 has an arc shape formed along the periphery of the barrel part 6.