The invention refers to a flyback transformer with a focus pack (100) mounted on a transformer case (14), said focus pack (100) arranging a circuit board (35) having a resistor circuit including a variable resistor for focus voltage supply mounted thereon. According to the invention the flyback transformer comprises an outer case (16) mounted on said transformer case (14) and an inner frame (22) formed inside and integrally with said outer case (16) for incorporating said circuit board (35); an opening path (23) formed between said outer case (16) and said inner frame (22) to be continuous from a side of said inner frame (22) to a bottom thereof; a shaft hole (25,26) extending through said outer case (16) and said inner frame (22), a retaining cylinder (28,29) adapted to be inserted into said shaft hole (25,26); and a rotatable operation shaft (32,33) adapted to be arranged into a cylinder hole.