Manufacturing a working portion and a joined portion through the same processing method provides a dental root canal treatment instrument allowing a simplified manufacturing process. A metal, rod-like main frame 11 includes a working portion 11a having a tapered point to be inserted in a root canal during use, and a joined portion 11b buried within a handle 12, continuing to the end on the wide side of the working portion. The working portion and the joined portion are formed through the same processing method. Injection of a synthetic resin to cover the joined portion 11b prepares the handle 12, thereby forming a dental root canal treatment instrument 10. The joined portion has at least one concave portion or convex portion in order to reinforce the joint with the handle.
