A surgical instrument includes an osteotomy guide portion configured to guide a cutting direction of a tibia, a supporting portion configured to support the osteotomy guide portion, a base housing to which the supporting portion is attached, a pair of ankle arms, an interlocking mechanism, and a center position marker portion. The pair of ankle arms are slidably mounted with respect to the base housing and arranged so that an ankle is sandwiched between the pair of ankle arms from both sides of the ankle. The interlocking mechanism is configured to interlock and slide the pair of ankle arms with respect to the base housing. The center position marker portion is configured to indicate a center position between the pair of ankle arms in a slidably moving direction of the pair of ankle arms.