A power supply device for a high frequency treatment instrument includes an active side detection circuit that acquires a first signal output from a treatment instrument-connecting terminal to the treatment instrument and a second signal returned from the treatment instrument to the treatment instrument-connecting terminal, a passive side detection circuit that acquires a third signal output from the treatment instrument-connecting terminal to the treatment instrument and passing through a return electrode to a return electrode-connecting terminal, and a processor that calculates a return loss as the second signal to the first signal and a first insertion loss as the third signal to the first signal and determines an abnormality occurrence location based thereon.