A surgical instrument (1, 1A) comprising: an insertion section configured to be capable of inserting into a body; first and second jaws (10, 50) provided at a distal end portion of the insertion section in order to grasp tissues and having a predetermined curved shape; a bonding section configured to bond the tissues grasped by the first and second jaws (10, 50); a guide section (24, 24A, 24B) formed along the predetermined curved shape and disposed at the first jaw; an actuation section having a pair of guided sections including a first guided section (34, 34B) and a second guided section (35, 35B), a base, and a blade section, the guided sections (34, 34B, 35, 35B) being engaged with the guide section (24, 24A, 24B) and being capable of contacting with the guide section (24, 24A, 24B), the guided sections (34, 34B, 35, 35B) being provided at positions separated from one another, the base being continuous to the guided sections (34, 34B, 35, 35B), and the blade section being attached to the base; and a connecting member including a dissection connecting member and a returning connecting member, wherein the dissection connecting member is configured to extend from a proximal end portion of the guide section (24, 24A, 24B) toward a distal end portion of the guide section (24, 24A, 24B), to be turned back at the distal end portion of the guide section (24, 24A, 24B), to extend toward the proximal end portion of the guide section (24, 24A, 24B), and to be coupled to the actuation section, wherein the returning connecting member is configured to be coupled to the actuation section, and wherein the first guided section (34, 34B) is supported by the guide section (24, 24A, 24B) at two opposite points (α1, α2), and the second guided section (35, 35B) is supported by the guide section (24, 24A, 24B) at two opposite points (β1, β2), characterized in that the guided sections (34, 34B, 35, 35B) are rotatable with respect to the base, the distal end of the dissection connecting member is connected to the first guided section (34, 34B), and the distal end of the returning connecting member is connected to the second guided section (35, 35B). an insertion section configured to be capable of inserting into a body; first and second jaws (10, 50) provided at a distal end portion of the insertion section in order to grasp tissues and having a predetermined curved shape; a bonding section configured to bond the tissues grasped by the first and second jaws (10, 50); a guide section (24, 24A, 24B) formed along the predetermined curved shape and disposed at the first jaw; an actuation section having a pair of guided sections including a first guided section (34, 34B) and a second guided section (35, 35B), a base, and a blade section, the guided sections (34, 34B, 35, 35B) being engaged with the guide section (24, 24A, 24B) and being capable of contacting with the guide section (24, 24A, 24B), the guided sections (34, 34B, 35, 35B) being provided at positions separated from one another, the base being continuous to the guided sections (34, 34B, 35, 35B), and the blade section being attached to the base; and a connecting member including a dissection connecting member and a returning connecting member, wherein the dissection connecting member is configured to extend from a proximal end portion of the guide section (24, 24A, 24B) toward a distal end portion of the guide section (24, 24A, 24B), to be turned back at the distal end portion of the guide section (24, 24A, 24B), to extend toward the proximal end portion of the guide section (24, 24A, 24B), and to be coupled to the actuation section, wherein the returning connecting member is configured to be coupled to the actuation section, and wherein the first guided section (34, 34B) is supported by the guide section (24, 24A, 24B) at two opposite points (α1, α2), and the second guided section (35, 35B) is supported by the guide section (24, 24A, 24B) at two opposite points (β1, β2), characterized in that the guided sections (34, 34B, 35, 35B) are rotatable with respect to the base, the distal end of the dissection connecting member is connected to the first guided section (34, 34B), and the distal end of the returning connecting member is connected to the second guided section (35, 35B).