To provide a dental file excellent in flexibility, cutting property and breakage resistance.SOLUTION: A dental file 10 comprises a work part 11 which is formed into a spiral state and whose cross section becomes smaller as advances to a tip end, and a cross section at an optional position of the work part 11 is formed into an almost rectangular form formed of one arc 40 and three lines 41, 42, 43. Two points of points of four corners of the almost rectangular form are arranged on a circle of a virtual circle 50 having the arc 40 as a part thereof and two points of them are arranged inside the virtual circle 50, and a center of the virtual circle 50 is arranged inside the almost rectangular form. A shortest length of a perpendicular of lengths of perpendiculars from the center of the virtual line 50 having the arc 40 as a part thereof to the three lines 41, 42, 43 is 45% or greater and 65% or smaller with respect to a radius of the virtual circle 50.SELECTED DRAWING: Figure 1【課題】柔軟性、切削性、耐破折性に優れた歯科用ファイルを提供する。【解決手段】本発明の歯科用ファイル１０は、螺旋状で先端に向かって断面が小さくなっている作業部１１を有し、その作業部１１の任意の位置での断面形状が１本の円弧４０と３本の線４１，４２，４３で構成される略長方形であり、その略長方形の四隅の点は、前記円弧４０を一部分とする仮想円５０の円上と仮想円５０の内側にそれぞれ２点ずつ配置されており、かつ前記略長方形の内側に仮想円５０の中心位置があり、前記円弧４０を一部分とする仮想円５０の中心位置から前記３本の線４１，４２，４３までのそれぞれの垂線の長さのうち最短の垂線の長さが、前記仮想円５０の半径に対して４５％以上６５％以下であることとする。【選択図】図１
