To facilitate insertion of an implant while retaining a cut in an opened state. There is provided a spreader 1 for high tibial osteotomy to be inserted into a cut made in the deformed femur or tibia of a patient with knee osteoarthritis, and to open the cut so as to make a space that allows insertion of an implant, comprising: two pairs of swinging members 2a/2b and 3a/3b which are respectively connected in a relatively swingable manner by hinge parts 6 and 7 disposed at the distal end and two opening/closing mechanisms 4 and 5 which open/close these two pairs of swinging members 2a/2b and 3a/3b respectively about the axes of the hinge parts 6 and 7, wherein the two pairs of swinging members 2a/2b and 3a/3b are detachably assembled in the axial directions of the hinge parts 6 and 7.
