A surgical instrument includes a rod unit extending in a first direction along a tibia of a human body, an attachment located at a first end of the rod unit that is attachable to an ankle joint, and a fastener located at a second end of the rod unit that is fasten-able to a proximal end of the tibia. The fastener has a through-hole extending in a second direction that crosses the first direction, and a pin that is arranged in the through-hole and that has a tip which is received in the tibia. The through-hole has an opening larger than a profile of the pin to allow the fastener to move about a portion of contact between the through-hole and the pin when the pin is received in the tibia.