An internal joint cavity expander 10 includes femur-side magnets 12, tibia-side magnets 18, and magnetic shield members 24. Each femur-side magnet 12 is a magnet having a plate shape curved into an arc and fixed to a femur 52. Each tibia-side magnet 18 is a magnet having a length smaller than the length of the femur-side magnet 12 and having a plate shape curved into an arc concentric with the arc of the femur-side magnet 12. Each tibia-side magnet 18 is fixed to a tibia 54. The pole disposed on the convex surface of each femur-side magnet 12 is the same as the pole disposed on the concave surface of the corresponding tibia-side magnet 18. The convex surface of each femur-side magnet 12 is disposed so as to face the concave surface of the corresponding tibia-side magnet 18. Each magnetic shield member 24 is disposed so as to cover the entirety of a surface of the corresponding femur-side magnet 12 and the corresponding tibia-side magnet 18 facing the skin throughout the range in which the femur-side magnet 12 moves relative to the tibia-side magnet 18. Each magnetic shield member 24 prevents the magnetism of the corresponding magnets 12 and 18 from leaking outside without hindering the movement of the femur-side magnet 12 relative to the tibia-side magnet 18.
