To secure grounding of a potential generator to protect a human body from danger of electrification by detecting leak current between each line of a commercial power source and an output, and connecting one line of the commercial power source judged as the grounding side with a grounding terminal.

Grounding line detection of a potential generator 1 acts when an output is set and when therapy is executed. That is, one line of input lines 12a and 12b is selected by a line switching part 14 and leak current between the selected one line and output terminal 1a and a grounding terminal 1b is detected by a leak current detecting part 15. Further, the unselected line from the input lines 12a and 12b is selected by the line switching part 14 and leak current between the output terminal 1a and the grounding terminal 1b is similarly detected by the leak current detecting part 15. The smaller one of the input lines 12a and 12b is judged as the grounding side and connected with the grounding terminal 1a causing the grounding is securely performed.