An internal joint cavity expander (10; 110; 210; 310) that is configured to be disposed in an incision in a natural knee joint consisting of a femur and a tibia on a left side and a right side of the knee joint to retain a space in the knee joint between the femur and the tibia, the internal joint cavity expander (10; 110; 210; 310) including, for each of the left side and the right side of the knee joint: a femur-side magnet (12; 312) that is adapted to be fixed to the femur when the internal cavity expander is disposed in the incision and has a shape of a plate curved into an arc so as to protrude toward the tibia, a tibia-side magnet (18; 218; 318) that is adapted to be fixed to the tibia when the internal joint cavity expander is disposed in the incision, wherein the tibia-side magnet (18; 218; 318) is shorter in length than the femur-side magnet (12; 312), and has a shape of a flat plate or a plate curved into an arc concentric with the arc of the femur-side magnet (12; 312), and a magnetic shield member (24; 124) that is adapted to be fixed to either the femur or the tibia, such that, when the internal joint cavity expander is disposed in the incision, the magnetic shield member (24; 124) is disposed so as to cover the entirety of a surface of the femur-side magnet (12; 312) facing a skin and the entirety of a surface of the tibia-side magnet (18; 218; 318) facing the skin throughout a range in which the femur-side magnet(12; 312) moves relative to the tibia-side magnet, and prevents the magnetism of both magnets (12; 312, 18; 218; 318) from leaking outside without hindering movement of the femur-side magnet (12; 312) relative to the tibia-side magnet (18; 218; 318), wherein, the femur-side magnet (12; 312) and the tibia-side magnet (18; 218; 318) are configured to be disposed on the left side and the right side of the knee joint when the internal joint cavity expander is disposed in the incision so as to constantly face each other and repel each other and so that the repulsive force acts in the vertical direction while the tibia-side magnet (18; 218; 318) and the femur-side magnet (12; 312) are moving relative to each other in response to bending and stretching movements of the knee joint. a femur-side magnet (12; 312) that is adapted to be fixed to the femur when the internal cavity expander is disposed in the incision and has a shape of a plate curved into an arc so as to protrude toward the tibia, a tibia-side magnet (18; 218; 318) that is adapted to be fixed to the tibia when the internal joint cavity expander is disposed in the incision, wherein the tibia-side magnet (18; 218; 318) is shorter in length than the femur-side magnet (12; 312), and has a shape of a flat plate or a plate curved into an arc concentric with the arc of the femur-side magnet (12; 312), and a magnetic shield member (24; 124) that is adapted to be fixed to either the femur or the tibia, such that, when the internal joint cavity expander is disposed in the incision, the magnetic shield member (24; 124) is disposed so as to cover the entirety of a surface of the femur-side magnet (12; 312) facing a skin and the entirety of a surface of the tibia-side magnet (18; 218; 318) facing the skin throughout a range in which the femur-side magnet(12; 312) moves relative to the tibia-side magnet, and prevents the magnetism of both magnets (12; 312, 18; 218; 318) from leaking outside without hindering movement of the femur-side magnet (12; 312) relative to the tibia-side magnet (18; 218; 318), wherein, the femur-side magnet (12; 312) and the tibia-side magnet (18; 218; 318) are configured to be disposed on the left side and the right side of the knee joint when the internal joint cavity expander is disposed in the incision so as to constantly face each other and repel each other and so that the repulsive force acts in the vertical direction while the tibia-side magnet (18; 218; 318) and the femur-side magnet (12; 312) are moving relative to each other in response to bending and stretching movements of the knee joint.