In a prosthetic leg installed with a hydraulic cylinder for adjusting bending and extension resistance of a knee joint portion, a buffer device is provided between a lower leg portion and a foot portion below the knee joint portion for displacing movably in opposite directions when the toe and the heel of the foot portion contact the ground, respectively. An extension passage and a bending passage are provided in the hydraulic cylinder, a toe grounding closing valve is provided in the bending passage, and a heel grounding closing valve is provided in the bending passage. The toe grounding closing valve and the heel grounding closing valve are connected in an interlocking manner by a closing valve connecting/operating member such that valve open/close orientations thereof are set in opposite directions. A grounding detection member is provided for detecting movable displacement of the buffer device and displacing movably when the toe or heel on the foot portion of the prosthetic leg contacts the ground, and a grounding transmission member is provided for transmitting the movable displacement of the grounding detection member to the closing valve connecting/operating member to cause the closing valve connecting/operating member to displace movably.