PURPOSE: To eliminate a parasitic device, by a method wherein an all-N-type field effect switching device is used as a rectifying device in the rectifying stage of a synchronous rectifying circuit. CONSTITUTION: In a synchronous rectifying type power supply apparatus, the input terminal of an inductor 22 of an output filter 20 is connected to the common tap of the secondary winding of an isolating transformer, a 1st switching device 42' is connected to the 1st output terminal 16a of the secondary winding, and a 2nd switching device 44' is connected to the 2nd output terminal 16b of the secondary winding. Schottky diodes 46 and 48 are connected in parallel to the devices 42' and 44'. A device 70 is an all-N-type semiconductor device which has a main part 72, an upper surface 74, a lower surface 76 and 3 N-type material layers 82-86 with respectively different impurity concentrations. Further, a trench 78 is extended from the upper surface 76 to the inside of the main part 72 to form a mesa structure 79 which has gate electrode material 94 in the trench and an insulating layer 95, etc. in the crest part. COPYRIGHT: (C)1990,JPO