PROBLEM TO BE SOLVED: To provide: a medical or dental handpiece which can increase the contact ratio between gears with their tooth thickness sufficiently ensured, provide stable engagement between the gears, increase durability of the gears, and rotary-drive a treatment tool at a high speed and a medical or dental handpiece which is easy for an operator to use during cutting of a tooth.SOLUTION: A grip 3 contains therein a first gear mechanism 1000 through which first and second rotary shafts 101, 201 are connected, and a second gear mechanism 2000 through which the second rotary shaft 201 and a third rotary shaft 301 are connected. Rotation is transmitted from the rotary shaft 101 through the gear mechanism 1000 to the rotary shaft 201, and from the rotary shaft 201 through the gear mechanism 2000 to the rotary shaft 301. The gear mechanisms 1000, 2000 comprise an internal gear of a large diameter which is secured to one of the rotary shafts and an internal gear of a small diameter which is secured to the other of the rotary shafts, the internal gears being engaged with each other. External gears are substantially enclosed within virtual cylindrical planes 1001, 2001 tangent to peripheral edges of the internal gears.COPYRIGHT: (C)2015,JPO&INPIT【課題】歯厚を十分に確保した状態で歯車間の噛み合い率の値を大きくでき、歯車間に安定した噛み合わせが得られ、歯車の耐久性を向上し、且つ治療工具を高速で回転駆動できる医科歯科用ハンドピースを提供する。また、歯牙切削時に術者が使用しやすい医科歯科用ハンドピースを提供する。【解決手段】グリップ3の内部に第1、第2の回転軸101,201を連結した第1の歯車機構1000と第2、第3の回転軸201,301を連結した第2の歯車機構2000を収容し、回転軸101から歯車機構1000を介して回転軸201に回転を伝達し、回転軸201から歯車機構2000を介して回転軸301に回転を伝達し、歯車機構1000,2000は一方の回転軸に固定された大径の内歯車と他方の回転軸に固定された小径の内歯車を噛み合わせて構成され、内歯車の周縁に接する仮想円筒面1001,2001の内側に外歯車が実質的に内包されている。【選択図】図2
