Dental root canal treatment instrument

To reliably follow a tip end portion of a root canal and prevent biting into a root canal wall. A dental root canal treatment tool includes a shaft portion (A), a transition portion (D) formed continuously to the shaft portion, and a working portion (B) formed continuously to the transition portion. The working portion is formed in such a tapered shape that the working portion is narrowed from the transition portion to a tip end. A cross section (IIa) of the working portion closest to a shaft portion side is in a parallelogram shape having a pair of opposing long sides (1a), a pair of opposing short sides (2a), sharp edges (5a) formed on a long diagonal line (3a), and obtuse edge (6a) formed on a short diagonal line (4a). A cross section (IIb) of the working portion in the vicinity of the tip end thereof is in a shape having a pair of sides (1b) formed continuously from the long sides (1a), a pair of sides (2b) formed continuously from the short sides (2a), lands (7) formed continuously from the sharp edges on the long diagonal line (3b) and increasing in length from the shaft portion side to the vicinity of the tip end, and obtuse edges (6b) formed continuously to the obtuse edges on the short diagonal line (4b).