A method of manufacturing an elbow by machining, comprising forming a hole in a material to be formed into the elbow, said forming of the hole comprising: forming a starting hole (11 -3) in the material by rough cutting of the material from two directions with a rotary cutting tool, the starting hole (11 -3) having a machining remainder remaining at least on a hole surface on a side which corresponds to an inner side of a bend of the elbow as a finished product; finishing a radially inner surface of the starting hole (11 -3) on one end side using a side cutter including an arc-shaped cutting edge at an outer periphery thereof and having an outer diameter smaller than a finished hole diameter, characterized in that the side cutter is inclined at a predetermined angle (θ4) relative to the material such that an end surface of the side cutter is more remote from an opening of the starting hole (11 -3) on the inner side of the bend of the elbow to be cut, and is cut into the surface of the starting hole (11 -3), while simultaneously revolving the side cutter and rotating the side cutter about a center axis of the side cutter, such that the hole has a completely circular cross section taken along a plane perpendicular to a center axis of the hole and perpendicular to the center axis of the side cutter, and an oval cross-section taken along any plane not perpendicular to the center axis of the hole and perpendicular to the center axis of the side cutter, whereby the side cutter moves along the surface of the hole, and finishing the radially inner surface of the starting hole (11 -3) on another end side by inserting the side cutter into the starting hole (11 -3) from the another end side of the starting hole, and revolving the side cutter along the hole surface to be finished while rotating the side cutter. forming a starting hole (11 -3) in the material by rough cutting of the material from two directions with a rotary cutting tool, the starting hole (11 -3) having a machining remainder remaining at least on a hole surface on a side which corresponds to an inner side of a bend of the elbow as a finished product; finishing a radially inner surface of the starting hole (11 -3) on one end side using a side cutter including an arc-shaped cutting edge at an outer periphery thereof and having an outer diameter smaller than a finished hole diameter, characterized in that the side cutter is inclined at a predetermined angle (θ4) relative to the material such that an end surface of the side cutter is more remote from an opening of the starting hole (11 -3) on the inner side of the bend of the elbow to be cut, and is cut into the surface of the starting hole (11 -3), while simultaneously revolving the side cutter and rotating the side cutter about a center axis of the side cutter, such that the hole has a completely circular cross section taken along a plane perpendicular to a center axis of the hole and perpendicular to the center axis of the side cutter, and an oval cross-section taken along any plane not perpendicular to the center axis of the hole and perpendicular to the center axis of the side cutter, whereby the side cutter moves along the surface of the hole, and finishing the radially inner surface of the starting hole (11 -3) on another end side by inserting the side cutter into the starting hole (11 -3) from the another end side of the starting hole, and revolving the side cutter along the hole surface to be finished while rotating the side cutter.