A power diode is disclosed wherein it is possible to lower on-voltage by expanding a conducting region at an on time. By applying negative voltage to a plate electrode when turning on a power diode, an inversion layer is formed in a front surface layer of an n drift region sandwiched between a p guard ring region and a p anode region, and the p guard ring region and p anode region are connected by the inversion layer, thereby causing one portion or all of the p guard ring region to function as an active region together with the anode region, and expanding an energization region, thus lowering on-voltage.