An ultrasonic treatment apparatus comprising: an electric power source configured to output a vibration generating electric power; an ultrasonic transducer configured to generate an ultrasonic vibration when the vibration generating electric power is supplied from the electric power source; a treatment section which is configured to perform a treatment by use of the ultrasonic vibration generated in the ultrasonic transducer; a jaw which is openable and closable relative to the treatment section, and which includes a contact section contactable with the treatment section in a state where the jaw is closed relative to the treatment section; and an impedance detecting section configured to detect an ultrasonic impedance value of the ultrasonic transducer with time, in a state where the vibration generating electric power is output from the electric power source, further comprising a gradual decrease detecting section configured to detect a gradual decrease start point at which the ultrasonic impedance value starts to gradually decrease on the basis of detection results in the impedance detecting section; a tentative peak value holding section configured to hold the ultrasonic impedance value at the detected gradual decrease start point as a tentative peak value; a peak judging section configured to judge whether or not the held tentative peak value is a target peak of a detection target by comparing, to the held tentative peak value, changes with time of the ultrasonic impedance value after the gradual decrease start point; and an ultrasonic control section configured to control an output state of the vibration generating electric power from the electric power source, the ultrasonic control section being configured to output the vibration generating electric power from the electric power source in a second ultrasonic output mode where incision performance provided by the ultrasonic vibration in the treatment section becomes smaller than that in a first ultrasonic output mode before a peak detection point on the basis of a determination in the peak judging section, when at least a prescribed time (ΔT') passes from the peak detection point at which the target peak is detected, wherein the impedance detecting section is configured to detect a vibration generating current and a vibration generating voltage in the ultrasonic transducer with time, and configured to detect the ultrasonic impedance value on the basis of the detected vibration generating current and vibration generating voltage. an electric power source configured to output a vibration generating electric power; an ultrasonic transducer configured to generate an ultrasonic vibration when the vibration generating electric power is supplied from the electric power source; a treatment section which is configured to perform a treatment by use of the ultrasonic vibration generated in the ultrasonic transducer; a jaw which is openable and closable relative to the treatment section, and which includes a contact section contactable with the treatment section in a state where the jaw is closed relative to the treatment section; and an impedance detecting section configured to detect an ultrasonic impedance value of the ultrasonic transducer with time, in a state where the vibration generating electric power is output from the electric power source, further comprising a gradual decrease detecting section configured to detect a gradual decrease start point at which the ultrasonic impedance value starts to gradually decrease on the basis of detection results in the impedance detecting section; a tentative peak value holding section configured to hold the ultrasonic impedance value at the detected gradual decrease start point as a tentative peak value; a peak judging section configured to judge whether or not the held tentative peak value is a target peak of a detection target by comparing, to the held tentative peak value, changes with time of the ultrasonic impedance value after the gradual decrease start point; and an ultrasonic control section configured to control an output state of the vibration generating electric power from the electric power source, the ultrasonic control section being configured to output the vibration generating electric power from the electric power source in a second ultrasonic output mode where incision performance provided by the ultrasonic vibration in the treatment section becomes smaller than that in a first ultrasonic output mode before a peak detection point on the basis of a determination in the peak judging section, when at least a prescribed time (ΔT') passes from the peak detection point at which the target peak is detected, wherein the impedance detecting section is configured to detect a vibration generating current and a vibration generating voltage in the ultrasonic transducer with time, and configured to detect the ultrasonic impedance value on the basis of the detected vibration generating current and vibration generating voltage.