A head (43) of a torque wrench (41, 61) includes protuberances (46a, 46b) each of which includes a slope (46a1, 46a2) the height of projection, from the head body (44), of which gradually increases along the circumference of the head body (44) and which is oriented in a direction of rotation during a rotating operation such that the ultrasonic probe (11) is tightened into the ultrasonic vibrator (6), and a stop surface (46a2, 26b2) extending in radial directions of the head body (44). The head (43) includes an elastically deformable portion (48) configured to cause at least one of two opposite side portions of the slit (45) of the head body (44) to be elastically deformed so that the slit (45) is widened until a preset value is reached by torque applied from the engaging portion (47a, 47b) to the engagement portion (32, 51) with the protuberance (46a, 46b) in mesh with an interlocking portion (34) of the engagement portion (32, 51).
