PURPOSE: To obtain a transformer iron core which is lessened in no-load loss and noise and improved in magnetic characteristics by a method wherein magnetic steel plates are laminated so as to set a T joint between an iron core middle leg and a yoke both formed of magnetic steel plates smaller in step width than a corner joint between an iron outer leg and the yoke.

CONSTITUTION: A core middle leg 1 is formed through such a manner that hexagonal magnetic steel plates 11 each possessed of right-angled tips at both its lengthwise ends are laminated making the magnetic steel plates 11 of the core middle leg 1 at a T joint 4 between the core middle leg 1 and a yoke 3 deviate from the magnetic steel plates 13 of the yoke 3 in step width B13. A core outer leg 2 is formed through such a manner that trapezoidal magnetic steel plates 12 each possessed of a 45°-angled tip which confronts the core middle leg 1 inward are laminated making the magnetic steel plates 12 of the core outer leg 2 at a joint corner 5 between the iron outer leg 2 and the yoke 3 deviate from the magnetic steel plates 13 of the yoke 3 in step width B23. At this point, the step width B13 is set smaller than the step width B23.