The medical device includes a vibration transmission member having a distal end portion, a proximal end portion provided on the transducer side, and an intermediate portion provided between the distal end portion and the proximal end portion, and the distal end portion A first cross-sectional area reduction portion that is provided and has a cross-sectional area of the tip portion with respect to a surface intersecting with the central axis of the vibration transmitting member smaller than a cross-sectional area of the intermediate portion with respect to a surface intersecting with the central axis； A node position closest to the tip of the ultrasonic vibration is positioned in a state where the vibration transmitting member is in resonance, and is provided on a side opposite to the first cross-sectional area reducing portion with respect to the central axis. A cross-sectional area of a portion of the half of the vibration transmission member that is opposite to the first cross-sectional area reduction portion and that intersects the central axis； The central axis of the half of the area-decreasing portion side And a second cross-sectional area decreasing portion be smaller than the cross-sectional area related to the intersecting plane.医療機器は、先端部と、前記トランスデューサ側に設けられる基端部と、前記先端部と前記基端部との間に設けられた中間部と、を有する前記振動伝達部材と、前記先端部に設けられるとともに、前記振動伝達部材の中心軸と交差する面に関する前記先端部の断面積を前記中心軸と交差する面に関する前記中間部の断面積よりも小さくする第１断面積減少部と、前記振動伝達部材が前記共振をした状態で前記超音波振動の最も前記先端部寄りの節位置が位置されるとともに、前記中心軸に対して前記第１断面積減少部とは反対側に設けられる第２断面積減少部であって、前記第１断面積減少部とは反対側の前記振動伝達部材の半部の前記中心軸と交差する面に関する断面積を、前記振動伝達部材の前記第１断面積減少部側の半部の前記中心軸と交差する面に関する断面積よりも小さくする第２断面積減少部と、を備える。
