Abstract: A surgical instrument for cutting and separating biological tissue by employing ultrasonic vibration, particularly for cutting bones, separating the periosteum, and cutting and separating a coherent mass such as a calcium mass. An ultrasonic vibration-transmitting tool (5) which is connected to an ultrasonic vibration source (4) and performs mechanical vibration at the frequency of the ultrasonic waves has a spoon-shaped working portion (7) which is provided with at least one of a blade-shaped portion (25) forming a predetermined angle with respect to the direction of the mechanical vibration at the frequency of the ultrasonic waves and a blade-shaped portion (23, 24) substantially parallel with the direction of the vibration, the spoon-shaped working portion being adapted to be brought into contact with the biological tissue, and a liquid passage (21) passing through the interior of the ultrasonic vibration-transmitting tool, the liquid passage having an opening (22) which opens into the working portion.