A surgical instrument includes a medical treatment tool and an attachment device to which the medical treatment tool is attachable. The medical treatment tool includes a distal end part operable in multiple degrees of freedom, an elongate element for operating the distal end part, a shaft which accommodates the elongate element and the distal end part is connected to, and engagement member connected to the elongate element. The attachment device includes: an engaging member engageable with the engagement member； a support portion supporting the engaging member； a housing accommodating the engaging member； and a release mechanism that releases an engagement between the engaging member and the engagement member. The housing includes a lid portion provided with the support portion. The release mechanism moves, in conjunction with an opening operation of the lid portion, the support portion to a position where the engagement is released.
