In an ultrasonic treatment instrument, a vibration generation section is connected to a probe from a proximal direction side in an inside of a case by coupling a drive assembly integrally and detachably to a handle assembly. An opposite extended portion is extended from the case in a state where the opposite extended portion is opposed to the movable handle on the proximal direction side with respect to the movable handle and a space between the opposite extended portion and the movable handle can be opened and closed, when the drive assembly is coupled to the handle assembly. A battery is located in the opposite extended portion, when the drive assembly is coupled to the handle assembly.