A prosthetic leg knee joint includes a thigh connection part that is provided on the thigh side, a lower leg part that is provided rotatably around a knee shaft connected to the thigh connection part, a four-bar link mechanism that sets an instant center S whose relative position with respect to the thigh connection part is substantially constant regardless of the relative position between the thigh connection part and the lower leg part, and a braking unit that brakes the movement of the lower leg part in accordance with the relative position between the instance center S set by the four-bar link mechanism and the lower leg part.