There is disclosed a knee brace having first and second arms constituting a pair of left and right arms fixed to the thigh bilateral sides along the user's femur, and third and fourth arms constituting a pair of left and right arms fixed to the shank bilateral sides along the user's tibia, wherein the lower ends of the first and second arms and the corresponding upper ends of the third and fourth arms are pivotally connected together to provide two pairs of upper and lower arms, so that the third and fourth arms are allowed to pivotally move with a pivotal part as the center correspondingly to the knee motion. Gears and guide grooves respectively having racks meshing with the gears and extending in parallel to a femur or tibia axial center are provided opposite to the ends of the arms constituting the pairs of left and right arms. The arms constituting the pairs of upper and lower arms are pivotally connected together such that gears are allowed to slide in the guide grooves while being in mesh with the racks.