A hinge member substantially tubular in shape, where a plurality of the hinge members make up a bending treatment instrument by being arranged in a row substantially coaxially with one another, the hinge member comprising: a recess formed along a radial direction of the hinge member at least on an outer edge of an end face on a proximal side in an axial direction; and a pair of projections formed along the radial direction of the hinge member, at least on outer edges of an end face on a distal side by protruding therefrom and by being shifted 90 degrees in a circumferential direction from the recess, wherein each of the projections protrudes by forming a substantially arc shape and the recess is formed into an arc shape corresponding to the projections so as to be able to come into sliding contact with the projections; wherein a bending-wire passage hole penetrating the hinge member in the axial direction is formed at a position at the projections avoiding sliding surfaces of the projections on the end face on the distal side with respect to the recess; the hinge member further comprising a device-wire passage hole penetrating an axial center portion; characterised in that the bending-wire passage hole is formed at opposite ends of the pair of the projections for a total of four; and the hinge member has a first bending axis extending along an opposing direction of the pair of projections and passing through substantially center of the arc shape, and a second bending axis extending along an extending direction of the recess and passing through substantially center of the arc shape, wherein the bending-wire passage holes do not intersect with either of the first bending axis and the second bending axis. a recess formed along a radial direction of the hinge member at least on an outer edge of an end face on a proximal side in an axial direction; and a pair of projections formed along the radial direction of the hinge member, at least on outer edges of an end face on a distal side by protruding therefrom and by being shifted 90 degrees in a circumferential direction from the recess, wherein each of the projections protrudes by forming a substantially arc shape and the recess is formed into an arc shape corresponding to the projections so as to be able to come into sliding contact with the projections; wherein a bending-wire passage hole penetrating the hinge member in the axial direction is formed at a position at the projections avoiding sliding surfaces of the projections on the end face on the distal side with respect to the recess; the hinge member further comprising a device-wire passage hole penetrating an axial center portion; characterised in that the bending-wire passage hole is formed at opposite ends of the pair of the projections for a total of four; and the hinge member has a first bending axis extending along an opposing direction of the pair of projections and passing through substantially center of the arc shape, and a second bending axis extending along an extending direction of the recess and passing through substantially center of the arc shape, wherein the bending-wire passage holes do not intersect with either of the first bending axis and the second bending axis.