An internal joint cavity expander includes femur-side magnets, tibia-side magnets, and magnetic shield-members. Each femur-side magnet has a plate curved into an arc and fixed to a femur. Each tibia-side magnet has length smaller than the femur-side magnet length and having a plate curved into arc concentric with the femur-side magnet arc. Each tibia-side magnet is fixed to a tibia. The pole on the convex surface of each femur-side magnet is same as the pole on the concave surface corresponding tibia-side magnet. The convex surface of each femur-side magnet faces the concave surface of corresponding tibia-side magnet. Each magnetic shield-member covers the surface of corresponding femur-side magnet and tibia-side magnet facing the skin throughout range in which the femur-side magnet moves relative to the tibia-side magnet. Each magnetic shield-member prevents magnetism of corresponding magnets and from leaking outside without hindering the femur-side magnet movement relative to the tibia-side magnet.