PURPOSE: To produce a thin magnetic alloy sheet having D.C. and A.C. magnetic properties by cold-rolling a hot rolled plate of Fe-Ni alloy at specific high draft to form a sheet and then carrying out the final heat treatment.

CONSTITUTION: An Fe-Ni alloy ('Permalloy(R)') is refined and cast, and the resulting ingot is forged and hot-rolled so as to be formed into a recrystallized structure of ≤50μm average crystalline grain size before cold rolling, which is then cold-rolled by means of high-degree working at a draft of ≥65%, preferably ≥80%, so as to be formed into a cold rolled sheet of <150μm sheet thickness. Subsequently, the above sheet is subjected to the final heat treatment where the sheet is held, e.g. in a nonoxidizing atmosphere of dry hydrogen, etc., at ≥800°C for ≥5min and cooled slowly down to the room temp. By this method, the extra thin Fe-Ni sheet having superior D.C. and A.C. magnetic properties can be produced.