A surgical instrument for performing treatment to a treatment target portion, the surgical instrument comprising: a cylindrical elongated member having a long axis; a treatment part connected to the elongated member; and a connection part that detachably connects the elongated member to the treatment part, wherein the connection part comprises: a first turning axis part provided to the elongated member; a second turning axis part provided to the treatment part; a first rolling guide part that is provided to the elongated member and includes a circular-arc-shaped part coaxial with the first turning axis part; a second rolling guide part that is provided to the treatment part and includes a rolling guide part having a circular-arc-shaped part coaxial with the second turning axis part, the rolling guide part rollingly contacting the first rolling guide part; an engaging part that brings the treatment part and the elongated member into an engaging state, the connection part is detachable between the first turning axis part and the second turning axis part, characterized in that the engaging part comprises a main unit that is connected to the first turning axis part and is capable of engaging with the second turning axis part; a ring-shaped member that is provided separately from the main unit, and binds the main unit together with the second turning axis part; and a locking member that switches between a bound state and a released state, wherein in the bound state, the ring-shaped member is arranged in a position where it binds the main unit and the second turning axis part, and in the released state, the ring-shaped member is arranged in a position deviated from the position where it binds the main unit and the second turning axis part. a cylindrical elongated member having a long axis; a treatment part connected to the elongated member; and a connection part that detachably connects the elongated member to the treatment part, wherein a first turning axis part provided to the elongated member; a second turning axis part provided to the treatment part; a first rolling guide part that is provided to the elongated member and includes a circular-arc-shaped part coaxial with the first turning axis part; a second rolling guide part that is provided to the treatment part and includes a rolling guide part having a circular-arc-shaped part coaxial with the second turning axis part, the rolling guide part rollingly contacting the first rolling guide part; an engaging part that brings the treatment part and the elongated member into an engaging state, the connection part is detachable between the first turning axis part and the second turning axis part, characterized in that the engaging part comprises a main unit that is connected to the first turning axis part and is capable of engaging with the second turning axis part; a ring-shaped member that is provided separately from the main unit, and binds the main unit together with the second turning axis part; and a locking member that switches between a bound state and a released state, wherein in the bound state, the ring-shaped member is arranged in a position where it binds the main unit and the second turning axis part, and in the released state, the ring-shaped member is arranged in a position deviated from the position where it binds the main unit and the second turning axis part.