An ultrasonic treatment device (2) has piezoelectric elements (31A-31D). Upon imparting an AC voltage (V) to the elements (31A-31D), an AC current (I) flows through the elements (31A-31D) and the elements (31A-31D) generate ultrasonic vibrations. The vibrations are transmitted to a treatment section (17). The treatment section (17) vibrates at an amplitude (U1), and produces frictional heat between the treatment section (17) and biological tissue (H). This heat coagulates and incises the biological tissue (H). There is a gradual increase in the ultrasonic impedance (Z=V/I) as the incision progresses. Z equals Z1 at a time (t1), and then gradually starts to decrease. Z decreases only by ε1real during an interval from when Z starts to decrease until an interval (ΔT1) elapses. When ε1real is equal to or greater than a pre-set reference (ε), it is determined that Z peaks at the time (t1). In this case, V decreases in a manner such that the amplitude (U) of the treatment section (17) decreases to U2. As a result, the frictional heat produced in the treatment section (17) decreases. The incision of the biological tissue (H) is completed before this time. Doing so prevents the generation of excessive frictional heat.Linvention concerne un dispositif de traitement par ultrasons (2) comportant des éléments piézoélectriques (31A-31D). Après application dune tension alternative (V) aux éléments (31A-31D), un courant alternatif (I) circule dans les éléments (31A-31D) qui produisent alors des vibrations ultrasonores. Les vibrations sont transmises à une section de traitement (17). La section de traitement (17) vibre à une amplitude (U1), et produit une chaleur de frottement entre la section de traitement (17) et un tissu biologique (H). Cette chaleur coagule et incise le tissu biologique (H). À mesure que lincision progresse, limpédance ultrasonore (Z = V/I) augmente progressivement. Z est égal à Z1 à un moment (t1), et commence ensuite à diminuer progressivement. Z ne diminue q
