In an energy treatment system, circuitry temporally detects an electrical property value of an ultrasonic transducer. After an elapse of predetermined time since a power supply outputs electric power by first output power, when determining that the value has started gradual decrease, causes the power supply to change the electric power from the first output power to second output power smaller than the first output power, and after the change to the second output power, when the electrical property has gradually increased, causes the power supply to change the electric power from the second output power to third output power larger than the second output power.