An ultrasonic treatment system, comprising: an element unit including a piezoelectric element (35A-35F) which is configured to generate an ultrasonic vibration by being supplied with electric power, the element unit having a proximal end and a distal end, and extending along a longitudinal axis from the proximal end to the distal end; a proximal-side fixing member abutting on the element unit from a proximal direction side; a distal-side fixing member abutting on the element unit from a distal direction side, the distal-side fixing member being configured such that the element unit is sandwiched between the proximal-side fixing member and the distal-side fixing member in a longitudinal direction (C1, C2) which is parallel to the longitudinal axis, and being configured to transmit the ultrasonic vibration, which is generated in the element unit, toward the distal direction side; and a vibration transmitting member to which a probe, including a treatment section in a distal portion thereof so as to treat a tissue by the treatment section, is connected from the distal direction side in a region located on the distal direction side with respect to the distal-side fixing member, the vibration transmitting member being configured to transmit the ultrasonic vibration, which is transmitted from the element unit through the distal-side fixing member, to the probe, thereby transmitting the ultrasonic vibration toward the treatment section in the probe; and an ultrasonic source unit configured to supply the electric power to the piezoelectric element (35A-35F) so that the element unit, the proximal-side fixing member and the distal-side fixing member vibrate in a predetermined frequency range (Δf), a most proximal node position, which is located most proximally among the node positions, being located in the element unit in a state in which the element unit, the proximal-side fixing member and the distal-side fixing member vibrate in the predetermined frequency range (Δf), characterized in that the distal side fixing member has a greater acoustic impedance than the proximal side fixing member. an element unit including a piezoelectric element (35A-35F) which is configured to generate an ultrasonic vibration by being supplied with electric power, the element unit having a proximal end and a distal end, and extending along a longitudinal axis from the proximal end to the distal end; a proximal-side fixing member abutting on the element unit from a proximal direction side; a distal-side fixing member abutting on the element unit from a distal direction side, the distal-side fixing member being configured such that the element unit is sandwiched between the proximal-side fixing member and the distal-side fixing member in a longitudinal direction (C1, C2) which is parallel to the longitudinal axis, and being configured to transmit the ultrasonic vibration, which is generated in the element unit, toward the distal direction side; and a vibration transmitting member to which a probe, including a treatment section in a distal portion thereof so as to treat a tissue by the treatment section, is connected from the distal direction side in a region located on the distal direction side with respect to the distal-side fixing member, the vibration transmitting member being configured to transmit the ultrasonic vibration, which is transmitted from the element unit through the distal-side fixing member, to the probe, thereby transmitting the ultrasonic vibration toward the treatment section in the probe; and an ultrasonic source unit configured to supply the electric power to the piezoelectric element (35A-35F) so that the element unit, the proximal-side fixing member and the distal-side fixing member vibrate in a predetermined frequency range (Δf), a most proximal node position, which is located most proximally among the node positions, being located in the element unit in a state in which the element unit, the proximal-side fixing member and the distal-side fixing member vibrate in the predetermined frequency range (Δf), characterized in that the distal side fixing member has a greater acoustic impedance than the proximal side fixing member.