Abstract: A high-frequency transformer for a microwave oven comprises: an E-shaped first core of ferrite of a low-voltage side; a primary winding wound around the first core over a coil bobbin; an E-shaped second core of ferrite of a high-voltage side disposed in opposition with the first core with a specific gap therebetween; a gap-filling insulation material interposed between the first and second cores; a secondary winding wound around the second core; and a grounding plate connected electroconductively between the first core and a bottom plate of the microwave oven, wherein the distance between the primary winding and the first core is greater than the gap between the two cores.