A pipe bend die unit comprising a bend die with a pipe-receiving groove (11, 21) of half-circular cross section formed on an outer peripheral surface of the bend die to receive a pipe to be bent, the bend die being rotatable about a rotary axis, and the bend die comprising: a clamp member having a first groove part of half-circular cross section on an outer peripheral surface of the clamp member with a fitting recess formed on the first groove part and extending in a peripheral direction by a first predetermined length on a planar surface perpendicular to the rotary axis; and a counter pressure member having a second groove part of half-circular cross section formed on an outer peripheral surface of the counter pressure member, and a fitting protrusion extending in a peripheral direction by a second predetermined length from a tip end portion of the second groove part, the fitting protrusion being positioned in the fitting recess so that the first and second groove parts (11, 21) combine to form the pipe-receiving groove (11, 21) of half-circular cross section, the counter pressure member and the clamp member being hingedly connected to one another so as to be rotatable relative to each other about the rotary axis, characterized in that a fitting portion of the fitting protrusion that is positioned in the fitting recess is located at a foreside in an advancing direction of the pipe relative to a position where a bending operation of the pipe is initiated, and a mating portion at which the first groove part of the clamp member and the second groove part of the counter pressure member mate is located at a backside in the advancing direction of the pipe relative to the position where the bending operation of the pipe is initiated, the clamp member has a circularly recessed portion forming the first groove part of half-circular cross section, the fitting recess extending in a peripheral direction on a planar surface perpendicular to the rotary axis, the fitting recess being located at a bottom center of the circularly recessed portion, the first groove part of half-circular cross section being continuous with the circularly recessed portion including a part of the fitting recess, and the counter pressure member includes a curved surface portion formed on opposite sides of the planar surface, the curved surface portion being configured to contact the circularly recessed portion, the curved surface portion possessing an arch center on an axis offset from the rotary axis and perpendicular to the planar surface in a direction spaced from the rotary axis, the second groove part on the outer peripheral surface of the fitting protrusion possessing the half-circular cross section, with a bottom center of the second groove part being provided on the planar surface perpendicular to the rotary axis, next to the curved surface portion. a clamp member having a first groove part of half-circular cross section on an outer peripheral surface of the clamp member with a fitting recess formed on the first groove part and extending in a peripheral direction by a first predetermined length on a planar surface perpendicular to the rotary axis; and a counter pressure member having a second groove part of half-circular cross section formed on an outer peripheral surface of the counter pressure member, and a fitting protrusion extending in a peripheral direction by a second predetermined length from a tip end portion of the second groove part, the fitting protrusion being positioned in the fitting recess so that the first and second groove parts (11, 21) combine to form the pipe-receiving groove (11, 21) of half-circular cross section, the counter pressure member and the clamp member being hingedly connected to one another so as to be rotatable relative to each other about the rotary axis, characterized in that a fitting portion of the fitting protrusion that is positioned in the fitting recess is located at a foreside in an advancing direction of the pipe relative to a position where a bending operation of the pipe is initiated, and a mating portion at which the first groove part of the clamp member and the second groove part of the counter pressure member mate is located at a backside in the advancing direction of the pipe relative to the position where the bending operation of the pipe is initiated, the clamp member has a circularly recessed portion forming the first groove part of half-circular cross section, the fitting recess extending in a peripheral direction on a planar surface perpendicular to the rotary axis, the fitting recess being located at a bottom center of the circularly recessed portion, the first groove part of half-circular cross section being continuous with the circularly recessed portion including a part of the fitting recess, and the counter pressure member includes a curved surface portion formed on opposite sides of the planar surface, the curved surface portion being configured to contact the circularly recessed portion, the curved surface portion possessing an arch center on an axis offset from the rotary axis and perpendicular to the planar surface in a direction spaced from the rotary axis, the second groove part on the outer peripheral surface of the fitting protrusion possessing the half-circular cross section, with a bottom center of the second groove part being provided on the planar surface perpendicular to the rotary axis, next to the curved surface portion.