PURPOSE: To provide a method for machining a transformer core, whereby a gap in a magnetic circuit can be formed accurately and readily and an inexpensive transformer core can be fabricated.

CONSTITUTION: This method for machining the core of a transformer comprises machining a plurality of magnetic circuit forming planes 28 in a core 18 for constituting a transformer, and forming a gap 26 in one of the plurality of magnetic circuit forming planes. A first machining wheel 16 is combined with a second machining wheel 14 that is greater in diameter than the first machining wheel 16, to machine a plurality of magnetic circuit forming planes of the core and the gap simultaneously.