PURPOSE: To make an entire core small and light and to reduce the cost thereof, by a method wherein a main core of a voltage adjustment autotransformer is constructed of an E-shaped core and an I-shaped core, while a control core is constructed of the E-shaped core, and these cores are assembled integrally by a fastening metal fitting.

CONSTITUTION: The core of a voltage adjustment autotransformer 10 is constituted by a main core 16 constructed of an E-shaped core 12 and an I-shaped core 14, which are laminates, and forming a main magnetic flux loop, a control core 20 constructed of a laminated E-shape core 18 joined magnetically to the bottom part of the core 16, and laminated I-shape shunt cores 22 and 24 for bypassing a part of a main magnetic flux loop line. In windows 12b and 12c of the core 12, a shunt winding 30, a step-up winding 32 and a step-down winding 36 are disposed in separation by the cores 22 and 24. In windows 18b and 18c of the core 18, a direct-current control winding 40 for bringing a part of the core 16 under saturable control through the core 18 is disposed, and these cores 16 and 20 are combined integrally by fastening metal fittings 42 and 44 made up of an angle material.