A power supply device for a high frequency treatment instrument to treat living tissue includes an output unit to supply high frequency power to the treatment instruments electrode, a distance information acquisition unit to acquire a distance between the living tissue and the electrode, a determination unit to determine whether the distance satisfies a first condition, and an output control unit to control the output unit so that its output is placed in a controlled state if the distance satisfies the first condition and so that the output is set to a first output level higher than an output level in the controlled state if a second condition is satisfied after the output is placed in the controlled state.
