The disclosed technology is directed to a transducer unit used in a treatment instrument that comprises an ultrasonic transducer configured to generate ultrasonic vibrations. A cylindrically-shaped casing is configured to hold the ultrasonic transducer therein along a central axis. The cylindrically-shaped casing includes an outer circumferential portion having respective circular arc portions and extension portions formed thereon. A virtual polygon is formed by connecting together a plurality of virtual points located at positions outside of the outer circumferential portion and is separated by a predetermined distance from the central axis when a cross-section intersecting the central axis at right angles and a first virtual distance between the central axis and each side of the virtual polygon are defined. The circular arc portions each includes, as a radius from the central axis, a first length greater than the first virtual distance but smaller than the predetermined distance.