A surgical instrument for a surgical assist robot, the surgical instrument comprising: a cable member having electric conductivity and formed into a cable-like shape; an outer shell having a shape covering a circumference of the cable member and having electric conductivity; a first electric insulating portion (30, 130) arranged in at least a part of a space between the cable member and the outer shell, wherein the first electric insulating portion (30, 130) is made of a material having a higher electric insulation property than the cable member and the outer shell; a second electric insulating portion arranged at a position, which exists between the cable member and the outer shell and which is adjacent to the first electric insulating portion (30, 130), wherein the second electric insulating portion is made of a material having a higher electric insulation property than the cable member and the outer shell; a projecting portion (50, 133) projecting from a projecting electric insulating portion, which is one of the first electric insulating portion (30, 130) and the second electric insulating portion, toward a recessed electric insulating portion, which is the other (30, 130; 40), wherein the cable member is inserted into the projecting portion (50, 133); a recessed portion (33, 42) formed in the recessed electric insulating portion, wherein the projecting portion (50, 133) is inserted into the recessed portion (33, 42); a leading end portion arranged to be separated from the recessed electric insulating portion and the outer shell, wherein the leading end portion has electric conductivity and is bendable using tension of the cable member, of which end portion is connected to the leading end portion; and a power supply cable, which is a member having a cable-like shape and which is arranged inside the first electric insulating portion (30, 130) and the second electric insulating portion, for supplying an electric power from an outside to the leading end portion, wherein the first electric insulating portion (30, 130) comprises a region, on a leading end portion side, forming an outer shape of the surgical instrument for the surgical assist robot, and a region, on an opposite side of the leading end portion side, arranged inside the outer shell. a cable member having electric conductivity and formed into a cable-like shape; an outer shell having a shape covering a circumference of the cable member and having electric conductivity; a first electric insulating portion (30, 130) arranged in at least a part of a space between the cable member and the outer shell, wherein the first electric insulating portion (30, 130) is made of a material having a higher electric insulation property than the cable member and the outer shell; a second electric insulating portion arranged at a position, which exists between the cable member and the outer shell and which is adjacent to the first electric insulating portion (30, 130), wherein the second electric insulating portion is made of a material having a higher electric insulation property than the cable member and the outer shell; a projecting portion (50, 133) projecting from a projecting electric insulating portion, which is one of the first electric insulating portion (30, 130) and the second electric insulating portion, toward a recessed electric insulating portion, which is the other (30, 130; 40), wherein the cable member is inserted into the projecting portion (50, 133); a recessed portion (33, 42) formed in the recessed electric insulating portion, wherein the projecting portion (50, 133) is inserted into the recessed portion (33, 42); a leading end portion arranged to be separated from the recessed electric insulating portion and the outer shell, wherein the leading end portion has electric conductivity and is bendable using tension of the cable member, of which end portion is connected to the leading end portion; and a power supply cable, which is a member having a cable-like shape and which is arranged inside the first electric insulating portion (30, 130) and the second electric insulating portion, for supplying an electric power from an outside to the leading end portion,